Introduction

What are modal auxiliary verbs?

The modal auxiliary verbs are will, would, shall, should, can, could, ought, may, might and must. Their grammar is different from that of other verbs: for example, they have no infinitives, participles or past tenses. For details, (see here). Some other verbs and expressions are used in similar ways to modals: have to, be able to, be supposed to, had better and need.

Meanings

We do not usually use modal verbs to say that situations definitely exist, or that particular events have definitely happened. We use them, for example, to talk about things which we expect, which are or are not possible, which we think are necessary, which we want to happen, which we are not sure about, which tend to happen, or which have not happened. This is a complicated area of grammar: each modal verb has more than one use; on the other hand, different modals can have very similar uses. Modal meanings fall into four groups.

1. degrees of certainty ((see here)

   Most modal verbs can be used to express degrees of certainty about facts, situations or events: for instance to say that something is certain, probable, possible or impossible.

   • It will rain tomorrow.
   • She should be here soon.
   • We may get a new car.
   • It can’t be true.

2. obligation, freedom to act, etc ((see here)

   Most modal verbs can also express various ideas relating to obligation, freedom to act, recommended behaviour, permission, willingness and similar matters. They are common in requests, suggestions, invitations and instructions.

   • Students must register by 30 January.
   • You should see the doctor.
   • If you will come this way …
   • Could I talk to you for a moment?

   Note that these meanings are usually expressed from the speaker’s point of view in statements, and from the hearer’s in questions. Compare:

   • You must go and see Anna. (I think it is necessary.)
   • Must you go and see Anna? (Do you think it is necessary?)

3. ability ((see here)

   Modal verbs, especially can and could, are used to say what people and things are able to do because of their knowledge, skill, design and so on.

   • She can speak six languages.
   • My old car could do 120mph.

4. typical behaviour ((see here)

   We can talk about what people and things typically or habitually do or did, using can, may, will, would and used to.

   • Dan can really get on your nerves.
   • Most evenings he’ll just sit watching TV.
   • A female crocodile may lay 30–40 eggs.
   • I used to play a lot of tennis.

Do you know what’s wrong with these, and why?

I must to water the plants. (see here)

It mustn’t be Emily at the door – she’s in Ireland. (see here)

You mustn’t work tomorrow if you don’t want to. (see here)

It can rain this afternoon. (see here)

May you go camping this summer? (see here)

I felt very hot and tired. I might be ill. (see here)

I might read what I liked when I was a child. (see here)

Next week’s exam must be easy. (see here)

Yesterday I should clean the whole house. (see here)

James should get back home yesterday. Has anybody seen him? (see here)

It can rain this afternoon. (see here)

I’m not sure where Emma is. She can be out shopping. (see here)

You have better hurry up. (see here)

‘The phone’s ringing.’ ‘I’m going to answer it.’ (see here)

‘There’s the doorbell.’ ‘I go.’ (see here)

I phone you tonight. (see here)

May everybody park here? (see here)

Yesterday evening Daniel could watch TV for an hour. (see here)

I could run 10km yesterday. (see here)

I can speak French at the end of this course. (see here)

He uses to smoke. (see here)

You used not to like him, used you? (see here)

note

For will as a future auxiliary, (see here).

For would in conditional structures, (see here), (see here).

For polite uses of would in ‘distancing’ structures, (see here).

For should in subordinate clauses (e.g. It’s funny she should say that), (see here).

For modal-like uses of need, (see here).

For modal-like uses of dare, (see here).