Syntax
A modal verb serves as an auxiliary to another verb, which appears in infinitive form (the bare infinitive, or the to-infinitive in the cases of ought and used as discussed above). Examples: You must escape; This may be difficult.
The verb governed by the modal may be another auxiliary (necessarily one that can appear in infinitive form – this includes be and have, but not another modal, except in the non-standard cases described below under Double modals). Hence a modal may introduce a chain (technically catena) of verb forms, in which the other auxiliaries express properties such as aspect and voice, as in He must have been given a new job.
Modals can appear in tag questions and other elliptical sentences without the governed verb being expressed: ...can he?; I mustn't.; Would they?
Like other auxiliaries, modal verbs are negated by the addition of the word not after them. (The modification of meaning may not always correspond to simple negation, as in the case of must not.) The modal can combines with not to form the single word cannot. Most of the modals have contracted negated forms in n't which are commonly used in informal English: can't, mustn't, won't (from will), etc.
Again like other auxiliaries, modal verbs undergo inversion with their subject, in forming questions and in the other cases described in the article on subject–auxiliary inversion: Could you do this?; On no account may you enter. When there is negation, the contraction with n't may undergo inversion as an auxiliary in its own right: Why can't I come in? (or: Why can I not come in?).
More information on these topics can be found at English clause syntax.
Read more about this topic: English Modal Verbs