When discussing auxiliary verbs, we should pay special attention to modal verbs and semi-modals. Without diving into details, let’s just make a quick overview of all the modals and semi-modals that we have.
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Modal Verbs
Modal verbs are used as auxiliaries. Like with other auxiliaries, we don’t insert the auxiliary DO in questions and negative sentences:
He can do it.
Can he do it?
He can’t do it.
Besides, they have no nonfinite forms. You can’t use them as infinitives or participles, for example. Instead, you should use other forms:
I want to *can swim. → I want to be able to swim.
They will *can do it. → They will be able to do it.
*Canning sing I took part in the singing contest. → Being able to sing I took part in the singing contest.
There are nine basic modal verbs in English: can, could, must, may, might, shall, should, will, would. They all express a wide range of meanings like ability, necessity, probability, obligation, permission and more.
Semi-modals
Semi-modals are verbs and verbal expressions that are not pure modals, but share some of their features. Here belong the following verbs and expressions: dare (to), need (to), ought to, used to, have to, had better, (have) got to, be supposed to, be going to. They have features of both modal verbs and lexical verbs. For example with some of them we use DO in questions and negatives:
Do I need to answer all those questions?
I didn’t dare (to) say anything.
Some semi-modals have nonfinite forms:
He will have to do it himself.