Formal characteristics of modal verbs

Can, could, may, might, must, shall, should, will, and would are 'pure' modal verbs. They:

are not inflected in the third person, e.g. He must go (not ‘He musts go’)

are followed by the 'bare infinitive', e.g. I must go (not I must to go)

are negated by the addition of 'n't or 'not', e.g. I can't, I cannot (not I don't can)

are inverted with the subject to form a question, e.g. Should I do it?

mostly have no past form (not I musted go)

Need and dare

We can use both need and dare as modal verbs - we generally use need as a modal verb when we are in a position of authority and able to give permission or remove obligation. We tend to use it in negative statements.

She needn't bring the files as long as she can remember the main details.

We use dare as a modal verb, primarily only in fossilised expressions (e.g. I dare say; How dare you...?)

However, we also use need and dare as main verbs (i.e. not modal):

She needed to explain the circumstances.

I didn't dare speak.

'Semi-modal' verbs

Ought

Ought is similar to the pure modal verbs, except that we use the full infinitive (i.e. with to) after it rather than the bare infinitive.

You oughtn't to cook vegetables so long.

We generally include to when we miss out repetition of the main verb.

I don't really want to go into work tomorrow but I ought to.

However, we leave out to in 'question tags'.

We ought to pack up soon, oughtn't we? (not oughtn't we to?)

Had better

Had better is similar to pure modal verbs except that it consists of two words, and we form the negative in a different way.

We generally use had better to give advice:

You 'd better check that the doors are all locked.

The negative form of had better is had better not (not hadn't better!)

You 'd better not go home till the fog clears.

Many people avoid had better in question forms, preferring to use, for example, should. However, if we do use had better in a question form, we place the subject after had (or hadn't).

Hadn't you better wear something warmer?

Have (got) to

Have (got) to is modal in meaning, but not in form.

We use have to to express necessity or obligation:

They'll have to take the whole course again.

We use the auxiliary do or did in questions and negative statements:

Do you have to do that?

They didn't have to check in.

In spoken English and in informed written English, many people use have got to as an alternative to have to.

Haven't they got to scrub the wall down before they begin painting it? (Don't they have to...?)

We use have got to mainly in the present tense, but it is possible to us< in the past tense:

They thought they'd got to sign the contract right there and then. (… thought they had to…)

In British English, the answer to Have you got to...? is usually Yes, I have/No, I haven't. In American English it is usually Yes, I do/No, I don’t.

Be able to

Be able to is modal in meaning, but not in form.We use be able to to express ability:

I'm sorry, he ' s not able to come to the phone just at the moment.

Have to, be able to, tense and use after other modal verbs

'Pure' modal verbs have no tense forms, and we generally use them to refer only to the present or the future.

We can use have to and be able to, on the other hand, in the full range of tense forms.

She 'll have to learn to drive if she moves to the country.

I wish I'd been able to persuade them to stay here longer.

We can also use have to and be able to after other modal verbs:

We might have to help.

They ought to be able to repair it.

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