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Modal verbs express modality, or the speaker's meta-comment on the sentence - his/her assessment of the statement, e.g., regarding its factuality, possibility, or necessity.

The following is an overview of modal verbs, largely drawing from Cowan (2008)[1]. These fall into pure modals (e.g., can, should, may) and semi-modals (e.g., have to, ought to).


1 General properties

Modals are a special type of verb that lack full lexical-semantic content, and are classified as function words (like auxiliary and linking verbs), rather than as full verbs and content words (cf. full verbs like make, remove, destroy). That is, their content is more functional or pragmatic, in that they express the speaker's evaluation or assessment of the situation, a sort of whole sentence-level modification -- i.e., modality. For example, a statement like Khalil must take the course expresses the speaker's opinion of necessity regarding taking the course.

A modal verb must take a following dependent verb (must go, should go) without the infinitive marker to, while semi-modals (or quasi-modals) must use the infinitive marker (have to go, ought to go). Modals fall into two general semantic categories: deontic (need, obligation) and epistemic (regarding the truth value of the situation).

Deontic: must, should, have to

Epistemic: might, may, can, could, would

However, it is not necessarily the case that a given modal is always one or the other. The verb must is primarily deontic (he must leave now), but has a secondary epistemic meaning (you must be joking), which is possible via implicature (=this must / has to be the reason for what you are saying). Likewise, can is primarily epistemic (I can do it), can take on a deontic meaning, e.g, for requesting permission in informal English (Can I go now?).

2 Word order of modals

be (+ -ing)
be/get (+PP)
You must have been dreaming
My house must have been robbed
He couldn't have gotten re-elected
It could have been raining
The dog shouldn't have been let out

Adverbs can come between the modal and the main verb

You couldn't have possibly gone there.
You must have really been famished.

3 Summary of modal meanings

ability can, could
admonition can't
advice might, could, should, had better, ought to
cause (CAUSATIVE) make, have
commands will!, shall! (formal)
future will, shall
habitual used to, would
inferred probability must, have to, ought to, should
intention be supposed to
necessity can't, must, need, have to, have got to, be to
obligation be to, have got to, have to, must, can't, should, ought to, be supposed to
offers shall
permission may, might, can, could, let
probability could, might, may, should, ought to, must, has to, will, would
recommendation had better, should
requests can, could, would, will (strong request), might (formal)
suggestion shall, ought to, might, could, let's [let us]
wish, desire would, would like, would like to, may, might

4 Pure modals

4.1 can

4.1.1 a) ability: "be able to do something”, "know how to do something"
  • I can lift over 50 pounds.
  • I can write backwards.

4.1.2 b) permission (an informal manner of asking permission)
  • Can I make a phone call?
  • Can I go to the bathroom?

4.1.3 c) possibility — possible facts, theory, disbelief
  • Your visa can be revoked if you break the law.
  • In a tornado, wind speeds can exceed 200 mph (320km/hr).
  • Anyone can make mistakes.
  • Who can that be at the door?
  • Can you really write backwards?

4.1.4 d) can't: disbelief
  • He lost the election? You can't be serious!

4.1.5 e) can't: obligation, , necessity, admonition (=must not)
  • You can't tell her that! She'll never speak to me again if you do!

PAST: was/were able to We weren't able to find it.

PERFECT: has/have been able to We haven't been able to find it.

FUTURE: will be able to We won't be able to find it.

4.2 could

Often like can, but more formal or polite in meaning; or more conditional or contrary to fact. In colloquial English, could have contracts to coulda.

4.2.1 a) possibility and ability (conditional)
  • What could I do with a million dollars? I could buy a Carribean island.
  • It could snow today.
  • I could be a great student, if I studied harder.

4.2.2 b) past ability (past, past habitual)
  • When I was young, I could swim from Florida to Cuba. Now I'm too old.
  • When I was in Germany, I could eat baklava every day at Turkish delis.

4.2.3 c) past conditional (in PERFECT tense)
  • She could have gone to the movies, but instead she played video games.
  • I could have learned Japanese in college, but instead I took Russian.

4.2.4 d) permission
  • Could I be excused to go to the bathroom?

4.2.5 e) advice, strong suggestion
  • Could you knock before entering my bedroom?
  • You know, you could knock on the door before you come into my bedroom.

4.2.6 f) past reported (indirect) speech

Could is the past of can in reported speech.

  • She said, "You can watch whatever you want to.”
  • She said you could watch whatever you want to.

4.2.7 g) couldn't = disbelief
  • He couldn't have committed such a horrible crime! He comes from such a good family.
  • She couldn't have stolen your wallet. She was with me at the time.

4.3 may

4.3.1 a) possibility
  • The President may not be telling the truth. He may have done some illegal or questionable things.

4.3.2 b) permission (more formal for permission than can)
  • May I go now? Yes, you may.

4.3.3 c) wishes
  • May the best person win. May the sun shine on you, and may the wind always be at your back.

4.3.4 d) may not = prohibition
  • You may not leave the room until the test is over, and the teacher has called time.

4.4 might

4.4.1 a) possibility

Expresses possibility that is more tentative, speculative, theoretical, remote, or uncertain than may.

  • I might visit you when I go to Chicago, but then I might not if I'm too busy. They may have met in New York, but I don't know if she was there at that time.

4.4.2 b) advice, suggestion

In the present tense, might implies a neutral or more polite suggestion. In the perfect tense, it can imply criticism.

  • I don't know what's wrong. But you might try shutting the computer down and rebooting. You ate all my birthday cake! You might have at least saved some for me! You went bungee jumping! You might have killed yourself!

4.4.3 c) very formal requests
  • Might I have a look at the contents of your bookbag?

4.5 will

a) future tenses: plans, intention, volition, prediction, general facts, etc.

  • I will go to Florida for vacation, if the weather is good.
  • It certainly looks like that lovely couple will get married someday.
  • The dog will run away if you don't take proper care of it.
  • I will be traveling Europe next month. (FUTURE PROGRESSIVE)
  • I will have finished the project by the time you come back. (FUTURE PERFECT)

4.5.1 b) habitual action

This a colloquial way of expressing present habitual.

  • Susan will come in here from time to time just to buy herbs and spices.

4.5.2 c) strong requests
  • Will you stop talking so loudly? Will you shut up?

4.5.3 d) strong commands
  • You will clean up your room right now!

4.5.4 e) strong probability
  • This is used to express probability about present or future events, which the speaker is very certain about.
  • That train will be the 6:30 Amtrak headed for St. Louis.

4.6 would

In informal English, would have contracts to would've, or more colloquially, woulda.

4.6.1 a) hypothetical condition or possibility (conditional)
  • He would sleep all day if you let him. What would you do if you were in this situation? I would tell her the truth about him, if I were you. ( = if I were in your position) We would have failed the class if you hadn't helped us.

4.6.2 b) past habitual (narrative)

Would forms a past habitual, a conversational equivalent of used to, used for narrating stories or sequences of habitual past events.

  • He would come in here every Saturday, buy a newspaper, talk with the clerk, and leave.

4.6.3 c) insistence

Would expresses the idea that a subject did something and insisted on doing it contrary to someone's wishes.

  • He would get into fights, even though we told him that we would get into trouble someday.

4.6.4 d) probability

Would can express probability that is very likely or certain.

  • That would be Fred. = That's probably Fred

4.6.5 e) wishes
  • I wish he would leave me alone.

4.6.6 f) requests
  • Would you stop following me?
  • Would you please stop talking? This is a library, and we're all trying to study.

4.6.7 g) past reported (indirect) speech

Would is the past of will in reported speech.

  • He said, "If you build it, they will come."
  • He said that if you build it, they would come.

4.7 shall

This is considered more formal that will, and is more common in British English than in American. For (a) and (b), shall is used only in first person (I, we); for (c), shall is only used in second person (you).

4.7.1 a) volition, intention, determination, prediction
  • We shall overcome!
  • According to the polls, I shall win the race quite easily.

4.7.2 b) offers and suggestions
  • Shall I help you with that? Shall we go out for coffee?
  • Shall we dance?

4.7.3 c) strong commands (formal)
  • You shall do as I say! You shall clean up your room!

4.8 should

4.8.1 a) strong advice or recommendation
  • You should see a doctor about that nasty skin rash.
  • You should take proper care of the dog, or it will run away.

4.8.2 b) inferred probability
  • What the speaker infers or believes to probably be true, based on what the evidence (“it is very likely that...”).
  • We've driven quite far. We should be able to see Chinatown by now.
  • The stove is fixed now, so you shouldn't have any more fires.

4.8.3 c) conditional (British English)
  • If you should change your mind, let us know. (British)
  • If you should change your mind, let us know. (American)

4.8.4 d) unfulfilled condition ( = reprimand, reproach)

In the perfect tense, should refers to unfulfilled action or conditions. Often they have a negative tone of reprimand or reproach. In colloquial, should have is contracted to shoulda.

  • You should have helped her. After all, she's your sister. You shouldn't have gone speeding. Now you have to go to traffic court and pay a $100 fine.

PAST: had to PERFECT: have/has been able to FUTURE: will have to

4.9 must

4.9.1 a) obligation, necessity, compulsion
  • You must not sleep in class. You must follow my instructions.

Notice how negation works:

  1. must not = it is necessary / obligatory that you not do it
  2. do not have to = it is not obligatory that you do it
  3. do not need to = it is not necessary that you do it
  • You must not forget to turn off the stove when you leave the house. You don't have to cook dinner tonight. You don't need to cook dinner tonight.

4.9.2 b) inferred probability

Expresses probable conclusion that the speaker draws.

  • You've had a long trip. You must be tired now. How did you get here so fast? You must have driven 90 miles an hour on the highway.

PAST: had to We had to operate.

PERFECT: has/have had to We've had to operate.

FUTURE: will have to We will have to operate.

5 Semi-modals

5.1 need

5.1.1 a) need to = must, should
  • We need to speak with you. ( = it is necessary that...) You need to get some rest. ( = should)

5.1.2 b) need = necessity (British)
  • Need I say more? You needn't do that now.

5.2 let

5.2.1 a) permission
  • Let me go! Let your brother out of the closet!

5.2.2 b) let's, let us: suggestion, proposal (to group)
  • Let's go to that new sci-fi movie.

5.3 ought to

Ought to conveys the same meaning as should and both terms are usually interchangeable. In colloquial, it contracts to oughta.

5.3.1 a) advise, suggestion
  • You ought to see a doctor about that cough.

5.3.2 b) inferred probability
  • We ought to be able to see some mountains by now. Their flight ought to have arrived by now.

5.3.3 c) unfulfilled condition (reproach, reprimand)
  • You ought to have helped her; after all, she's your sister.
  • You ought not to let him get away with that.

5.4 used to

5.4.1 a) past habitual action or state
  • I used to go to the movies fairly regularly, but nowadays I'm just too busy.

5.5 had better

Had better expresses strong recommendation, similar to should, but is more colloquial. It is often contracted: I'd better, you'd better, s/he'd better, etc. British English also uses the expression had best.

5.5.1 a) strong recommendation
  • You'd better take your umbrella; it's going to rain.

5.6 have to

Have to is often a colloquial equivalent of must; it also substitutes for must in tense other than present tense, since must can only be used in simple present tense. It is pronounced [hæf tu:] ~ [tʊ] ~ [tə], as opposed to the regular verb have [hæv].

5.6.1 a) obligation, necessity, compulsion
  • You have to finish your homework before you do anything else. I had to leave town on an emergency. I've had to spend every weekend in the library this semester. I will have to take summer classes so I can graduate on time.

5.6.2 b) inferred probability
  • The temperature outside has to be below freezing. The lake is frozen over.
  • The temperature yesterday had to have been below freezing — all our garden plants are dead.
  • The temperature outside would have to be below freezing for the schools to close.

5.6.3 have got to

Have got to is a more colloquial form of have to, and is used in present tense only. It is often contracted to (‘ve) gotta. a) obligation, necessity
  • I've got to go to the bathroom. I gotta go home now.

5.7 be able to

This emphasizes the internal ability of the agent. It is also used for non-present tenses of can.

  • I am able to lift 50 kg. (more emphatic than can)
  • We were able to replicate the experiment. (past tense)

5.8 be to

Be to indicates necessity or obligation, somewhat like should, and sometimes implies an imperative meaning. Its use is considered rather formal.

5.8.1 a) obligation
  • You are to stand here and guard the store from any would-be robbers. I am to go to the testing center tomorrow morning at 10am.

5.8.2 would like + NOUN, would like to + VERB a) desire, wish (conditional or polite)
  • I would like a cheeseburger.
  • I would like to eat some Chinese food tonight.
  • I would like to go, if the weather is nice.

5.9 make

5.9.1 a) Causative

The subject causes something to happen.

  • I should make you clean this room immediately.

5.10 have

5.10.1 a) Causative

Slightly weaker in meaning than the make-causative, in the have-causative, the subject brings about or arranges for something to happen.

  • A good teacher has his/her students do lots of homework.

5.10.2 b) Experiential

The subject is not the doer (agent) of the agent, or even the recipient (object) of the action, but merely the experiencer of the action.

  • We had all our PC's go bad on us because of a virus.

6 Comparisons

6.1 Must versus should, ought to

Must is used to express a strong obligation (like a moral or social obligation, or otherwise some kind of obligation that is an external force); should and ought to express strong advisability or warning. So in the following sentence, must would sound odd; should or ought to would be better here.

  1. For this reason, online banking must be quick and simple.
  2. For this reason, online banking should be quick and simple.

Here, it would be better to say that it “should be quick and simple,” as a strong request or advice to someone; it would be hard to view this as some type of social or moral obligation that banks should fulfill (unless, e.g., one is speaking as a banking analyst critiquing the operations of one’s own banking company from a social or ethical perspective).

6.2 Can vs. could

The verb can puts more emphasis on something possible, though perhaps requiring effort or involving difficulty in achieving it. The verb could is more hypothetical in meaning. Note: the meaning of could is not primarily past tense, and its main use is not as a past tense of can. It historically developed in English as a past tense of can, but its main use in modern English is for hypothetical situations or conditional (if-type clauses) clauses. For example, trying to use could as a past tense form of can / be able to in this sentence sounds awkward or ambiguous.

  • We could replicate the experiment with subjects from a different country.

This is ambiguous, because native English readers would first interpret this as a hypothetical statement – “hypothetically, we might try to replicated it” or “hypothetically, we might be able to replicate it.” Rather than using could here, it would be better to either use was able to to emphasize the idea of ability, or to delete it altogether and use just the main verb, and/or the main verb modified by an appropriate adverb.

  1. We were able to replicate the experiment with subjects from a different country.
  2. We replicated the experiment with subjects from a different country.
  3. We successfully replicated the experiment with subjects from a different country.

However, in the negative, it is less ambiguous, so could not would be more common and clearer. Other cases where could is used as a non-ambiguous past tense form might be with appropriate temporal (time) clauses. Otherwise, could is more commonly used in conditional statements, for hypothetical and speculative statements, or for asking permission.

6.3 May vs. might

The verb may expresses as its core meaning the idea of possibility.

  • He might be lying to us.
  • This might be an interesting film.

An extended meaning is for expressing permission, but this is rather formal.

  1. May I leave now? (very formal)
  2. Could I leave now? (formal)
  3. Can I leave now? (informal)
  1. You may leave / may not leave now. (more formal)
  2. You can / cannot leave now. (more informal) (adapted from Cowan, 2008, p. 299)

The verb might indicates possibility, but with a sense that is more remote or less certain than may.

  • He might agree to it (but then, he might not).

It can also express speculation, suggestion, or implied criticism; in very formal British English, it can be used for requests.

  1. She might have bought it in Mongolia, but I’m not sure if she’s been there. [speculation]
  2. I’m not sure what the problem is, but you might try putting the RAM chip in a different slot. [suggestion]
  3. You might have asked if someone else wanted cake before eating the last piece! [criticism]
  4. Might I have another glass? [British, permission]

The forms may have, might have, could have are used for speculating about the past.

  1. He may have escaped (but we’re not sure).
  2. He might have escaped (but not so likely).
  3. He could have escaped (it’s possible, but we don’t know yet).

7 See also

Modal exercises

7.1 References

  1. Cowan, Ron.(2008).The Teacher's Grammar of English. Cambridge University Press.