Modal verbs are used to express ideas such as ability, necessity, permission, and possibility.

There are not many modal verbs: can, could, dare*, need*, may, might, must, shall, should, will, would. There are also modal constructions: be able to, ought to, be allowed to.

Modals always come first in a verb‎ formation, before other auxiliaries and the main verb:

{modal} + [auxiliary] + {main verb}

N.B. Modal verbs do not take a final -s for the third person singular. The verb which follows is always in the infinitive form:

He might be…

They should stay…

She could have…

Use

Modal verbs are used in various ways to express shades of meaning, primarily ability, necessity, permission, and possibility. They are also used in making offers‏‎ and requests‏‎.

Ability

Here modals are used to describe the ability (or not) to do something:

They can’t speak fluently.

He could beat the record easily.

See the main article, Modals – Ability‏‎.

Necessity

Modals here are used to show what is necessary or required to happen.

He must stop.

They should stop.

See the main article, Modals – Necessity‏‎.

Permission

Here modals tell us whether we are allowed to do something or not (i.e. giving orders).

You cannot leave yet!

Yes, you may get down from the table.

See the main article, Modals – Permission.

Possibility

Here we’re talking about what may or may not happen and our opinion about the chances of it happening.

He will win the Olympic gold this year.

She may be upstairs, I’m not sure.

See the main article, Modals – Possibility.

Form

We can use them with the present participle‏ -ing form to show continuous modality:

{modal} + {be} + {-ing}

He might be working late.

They should be coming soon.

He must be having a party.

We can use them with the past participle to show past modality:

{modal} + {have} + {past participle}

He might have been robbed.

They should have seen him.

He must have gone away.

We can use be able to, be allowed to and have [got] to after modals:

{modal} + {phrase}

I will be able to…

She might not be allowed to…

They should have to…

Note that we do not use these in continuous forms.

* DARE is regarded as a marginal modal verb because of its restricted usage – either in negative polarity or in the idiomatic “I dare say…” form. However, both structurally and semantically it does function within the system of modality, although in a limited way.

* NEED can be used as an ordinary verb, meaning “must have” e.g. I need a new coat. But in British English, NEED can also be used as a modal verb, e.g. You needn’t pay for my dinner. In this case, there is no “s” with the 3rd person singular, and questions and negatives are made without “do” e.g. Need we re ally go now?