Modal Verbs

Modal Verbs express necessity, ability, permission, or probability. Examples include 'can', 'must', and 'should'.

Modal verbs: an introduction

The modal verbs of English are as follows:

can, could, may, might, will, would, must, shall, should, ought to

Modals differ from ordinary verbs in three respects.

They take no 's' in the third person singular.
They form questions by inversion, so 'she can go' becomes 'can she go?'.
They are followed directly by the infinitive of another verb, without 'to'.

Probability

Modals express how certain the speaker is that something happened, is happening or will happen. Grammars describe these as modals of deduction, speculation, certainty or probability.

For example:

It's snowing, so it must be very cold outside.
I don't know where Daniel is. He could have missed the train.
This bill can't be right. £200 for two cups of coffee!

Ability

'Can' and 'could' express skill or ability. For example:

She can speak six languages.
My grandfather could play golf very well.
I can't drive.

Obligation and advice

'Must' and 'should' state that something is necessary or unnecessary, and they also give advice. For example:

Children must do their homework.
We have to wear a uniform at work.
You should stop smoking.

Permission

'Can', 'could' and 'may' request and grant permission. Modals also state that something is not allowed. For example:

Could I leave early today, please?
You may not use the car tonight.
Can we swim in the lake?

Habits

'Will' and 'would' express habits, whether present or past. For example:

When I lived in Italy, we would often eat in the restaurant next to my flat.
Daniel will always be late!

Past modals

The past modals 'could have + past participle', 'should have + past participle' and 'would have + past participle' are a frequent source of confusion, and a full section is devoted to them below.

Modal verbs of probability

These modals, also known as modals of deduction, speculation or certainty, express a supposition. The verb chosen reflects the degree of the speaker's confidence.

Talking about the present

must / might / could / may / can't + infinitive

For example:

I am waiting for Marina with another friend, Marcus.

I ask: 'Where is Marina?'

Marcus guesses:

She must be on the bus. (I'm fairly sure this is a good guess)
She might come soon. (maybe)
She could be lost. (maybe)
She may be in the wrong room. (maybe)
She can't be at home. (I'm fairly sure this isn't true)

Note that the opposite of 'must' in this use is 'can't'.

Will / won't

'Will' and 'won't' express a high degree of certainty:

She'll be at work now.

Should / shouldn't

Should and shouldn't are used to make an assumption about what is probably true, if everything is as we expect:

They should be there by now.
It shouldn't take long to drive here.

This use of should is not normally applied to unwelcome events. 'Will' is preferable there:

The underground will be very busy now (not: 'should be').

Can

Can is used for something that is generally possible, something we know sometimes happens:

Prices can be high in London.

Can is not used to talk about specific possibilities:

He could be on the bus (not: 'can be').

Talking about the past

must / might / could / may / can't + have + past participle

must have + past participle
might / might not have + past participle
could / couldn't have + past participle
may / may not have + past participle
can't have + past participle

For example:

You: Where was Marina last night?

Marcus:

She must have forgotten about our date.
She might have worked late.
She could have taken the wrong bus.
She may have felt ill.
She can't have stayed at home.

Will / won't + have + past participle

Will and won't / will not + have + past participle are used for past certainty (compare with present use of 'will' above):

The parcel will have arrived before now.

Should + have + past participle

Should + have + past participle can be used to make an assumption about something that has probably happened, if everything is as we expect (compare with present use of 'should' above):

The train should have left by now

Could

We can use could + infinitive to talk about a general possibility in the past (compare with the use of 'can' above):

Prices could be high in the sixteenth century.

This is not used to talk about specific possibilities in the past (instead we use could + have + past participle):

He could have been working late (not: 'could be'. As this is a specific possiblity, 'could be' is present tense)

Modal verbs of ability

Ability in this sense covers two distinct things.

The first is general ability: something that, once learned, can be done at any time, such as reading, swimming or speaking a language.

The second is specific ability: what one is or is not able to do in a particular situation, such as lifting something heavy or finding a place one is looking for.

Present

can / can't (for both general and specific ability)

Should + have + past participle can be used to make an assumption about something that has probably happened, if everything is as we expect (compare with present use of 'should' above):

I can play the piano.
She can speak English.
He can't drive. He's too tired.
We can't come now.

Past

could / couldn't (for general ability)

I could read when I was four.
She could speak French when she was a child, but now she has forgotten it.
He couldn't dance at all until he took lessons.
My grandfather couldn't swim.

was able to / couldn't (for specific ability)

When the computer crashed yesterday, I was able to fix it.(not 'I could fix it')
She was able to pass the exam, even though she hadn't studied much.(not 'she could pass')
He called us because he couldn't find the house.
I couldn't open the window.

could + have + past participle (an ability someone had in the past, but didn't use)

I could have played the piano well but I didn't practise enough.
We could have come earlier.
She could have studied law, but she preferred to become a secretary.

Future

will / won't be able to (general ability)

At the end of the course, you will be able to make your own website.
He won't be able to speak Japanese in a week! It will take months.

can / can't (specific ability)

I can help you tomorrow
I can't come to the party

Modal verbs of obligation

Obligation, meaning something one is required to do, is expressed by have to + infinitive, must + infinitive and should + infinitive.

Present

Affirmative

Negative

have to / don't have to

strong obligation (possibly from outside)

Children have to go to school.

(sometimes 'have got to')

no obligation

I don't have to work on Sundays.
You don't have to eat anything you don't like.

must / mustn't

strong obligation (possibly based on the speaker's opinion)

I must study today.

negative obligation

You mustn't smoke here.

should / shouldn't

mild obligation or advice

You should save some money.

mild negative obligation or advice

You shouldn't smoke so much.

Be careful about the difference between mustn't and don't have to!

Mustn't indicates that something is forbidden or ill-advised:

You mustn't eat so much chocolate, you'll be sick

Don't have to indicates that something is not required, though there is no objection to doing it:

I don't have to get up early at the weekend(of course, if I want to get up early, that's fine, but I can stay in bed if I want).

Past

Affirmative

Negative

had to / didn't have to

obligation in the past

I had to wear a school uniform when I was a child.

no obligation in the past

We didn't have to go to school on Saturdays.

must

changes to 'had to'

not used

should have + pp / shouldn't have + pp

a past action which didn't happen: the advice / regret is too late

You should have gone to bed earlier, now you have missed the train.

a past action which didn't happen: the advice / regret is too late

You shouldn't have taken that job., it was a bad idea.

Note that 'must have done' is a modal of deduction or speculation, not an expression of past obligation. For example: Marina must have left. Her coat's not here.

'Could have', 'should have' and 'would have'

These past modals are hypothetical in force. They refer to what did not in fact happen.

Could have + past participle

Could have + past participle indicates that something was possible in the past, or within one's power, but was not done.

I could have stayed up late, but I decided to go to bed early.
They could have won the race, but they didn't try hard enough.
Marina could have bought the book, but she borrowed it from the library instead.
He could have studied harder, but he was too lazy and that's why he failed the exam.

Couldn't have + past participle indicates that something was not possible in the past, however much one might have wished it.

I couldn't have arrived any earlier. There was a terrible traffic jam (= it was impossible for me to have arrived any earlier).
He couldn't have passed the exam, even if he had studied harder. It's a really, really difficult exam.

The pattern could have + past participle also ventures a supposition about something that happened in the past. Whether the supposition is correct remains unknown; it is offered as an opinion about what may have occurred.

Why is Daniel late?

He could have got stuck in traffic.
He could have forgotten that we were meeting today.
He could have overslept.

The pattern might have + past participle carries the same meaning:

He might have got stuck in traffic.
He might have forgotten that we were meeting today.
He might have overslept.

Should have + past participle

Should have + past participle names a course of action that would have been advisable but was not taken. Addressed to another person it amounts to advice about the past; applied to oneself it expresses regret.

Shouldn't have + past participle names a course of action that was inadvisable but was taken all the same.

I should have studied harder! (= I didn't study very hard and so I failed the exam. I'm sorry about this now.)
I should have gone to bed early (= I didn't go to bed early and now I'm tired).
I shouldn't have eaten so much cake! (= I did eat a lot of cake and now I don't feel good.)
You should have called me when you arrived (= you didn't call me and I was worried. I wish that you had called me).
Daniel should have left early, then he wouldn't have missed the plane (= but he didn't leave early and so he did miss the plane).

The same pattern, should have + past participle, refers to something the speaker assumes has already happened in the ordinary course of events. Certainty is lacking, which is precisely why 'should have' is chosen rather than the present perfect or the past simple. 'By now' frequently accompanies it.

His plane should have arrived by now (= if everything is fine, the plane has arrived).
Daniel should have finished work by now (= if everything is normal, Daniel has finished work).

We can also use this to talk about something that would have happened if everything was fine, but hasn't happened.

Elena should have arrived by now, but she hasn't.

Would have + past participle

Part of the third conditional.

If I had had enough money, I would have bought a car (but I didn't have enough money, so I didn't buy a car).

Because 'would' (and will) can also be used to show if you want to do something or not (volition), we can also use would have + past participle to talk about something you wanted to do but didn't. This is very similar to the third conditional, but we don't need an 'if clause'.

I would have gone to the party, but I was really busy. (= I wanted to go to the party, but I didn't because I was busy. If I hadn't been so busy, I would have gone to the party.)
I would have called you, but I didn't know your number. (= I wanted to call you but I didn't know your number, so I didn't call you.)
A: Nobody volunteered to help us with the fair. B: I would have helped you. I didn't know you needed help. (= If I had known that you needed help, I would have helped you.)

Using 'would'

We can use subject + would + infinitive (I would go) or subject + would + have + past participle (I would have gone).

'Would' has a considerable range of uses. In many of them it functions as the past-tense counterpart of 'will'.

Both 'had' and 'would' contract to 'd, but only 'would' takes a bare infinitive. 'Had' is followed by a past participle or by 'to + infinitive'.

The past of 'will' in reported speech

'Will' in direct speech normally becomes 'would' in reported speech.

Direct speech: I will be there at 6pm.
Reported speech: She said that she would be there at 6pm.

Direct speech: We won't see you at the party.
Reported speech: They said that they wouldn't see us at the party.

Willingness in the past

Willingness, meaning a readiness to do something, is expressed by 'will' in the present. 'I will help you' amounts to an offer, or to an expression of gladness to help. 'Would' carries the same sense in the past. The construction is most often negative, taking 'won't' for the present and 'wouldn't' for the past.

His mother won't let him go to the party.
His mother wouldn't let him go to the party.

Habits in the past

When we were children, we would go to the beach every summer.
When I lived in Japan, I would take Japanese lessons three times a week.

The meaning is close to that of 'used to + infinitive'.

(We can also use 'will' for typical behaviour or habits in the present, though this is much less common than using 'would' for the past. An example in the present is 'she'll talk and talk for hours!')

Requests

Requests take either 'will' or 'would'. The meaning is identical, but 'would' is the more polite of the two.

Will you please help me?
Would you please help me?

The second conditional

The second conditional takes 'would + infinitive'.

If I had enough money, I would travel around the world.
If we lived in Madrid, we would study Spanish.

The third conditional

The third conditional takes 'would + have + past participle'.

If I had gone to bed earlier, I wouldn't have been so tired.
If the train hadn't been late, we would have arrived on time.

With 'wish'

I wish that he wouldn't smoke so much.
I wish you would come to see me more often.

With set phrases to talk about what we want

'Would' occurs in a number of set phrases, all of which retain some sense of willingness.

I would like some coffee.
She would rather go to the cinema.
We would prefer to leave immediately.
Would you mind helping me?