English grammar - Roger Berry 2012

B6.4 Marginal and semi-modals
B6 Modal auxiliaries
Section B Development

Two groups of words are closely associated with the modals: the marginal modals, need, dare and ought to, and the semi-modals. Need and dare are said to be marginal as modals because they sometimes behave like modals, i.e. they can form interrogatives and negatives without do, have no -s for third person singular, and are followed by a bare infinitive (see A7):

Need he do that? No, he needn’t.

However, this is rare; they are much more common as (main) verbs:

Does he need to do that? No, he doesn’t need to.

And in the positive they are not used without to: He needs to hurry not ’He need hurry’.

Ought to is quite common as an alternative to should for weak obligation. It is included here because it can also behave like a modal, with to omitted:

We ought to go, oughtn’t we?

The semi-modals are so-called because they cover some of the meanings of the modals, such as futurity, obligation, past habits, permission and ability. And they are all followed by the infinitive (sometimes with to, sometimes without):

I am unable to take your call now.

After school we used to go down to the shopping centre.

You’d better apologise.

However, they are structurally different; in fact, they consist of a number of different structures, some of them involving primary auxiliaries. Here is a list of expressions commonly recognised as semi-modals:

be going to (see A5 and D5), used to, have to (have got to)

had better, would rather

be (un)able to, be allowed to, be supposed to

Sometimes they are used when a modal would be impossible:

I would like to be able to come.

’I would like to can come’ is wrong because can cannot be used as an infinitive. And have to can be used as a distancing equivalent of must in reported speech:

I must go. He said he had to go.

As function words the finite forms of primary auxiliaries often have weak stress and can have contractions in non-formal writing (e.g. I am becomes I’m). Of the modal auxiliaries only will and would have contractions: -’ll and -’d. See A7 for more on contractions.

NON-STANDARD FORMS

A number of the semi-modals have alternative, non-standard (written) forms, reflecting their pronunciation in casual speech:

I’m gonna, I’ve gotta, We oughta

as well as the dropping of the auxiliary:

I gotta, You betta/better