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 word can combine with not forms 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. Past forms The preterite (past) forms given above (could, might, should, and would, corresponding to can, may, shall, and will, respectively) do not always simply modify the meaning of the modal to give it past time reference. The only one regularly used as an ordinary past tense is could, when referring to ability: I could swim may serve as a past form of I can swim. All the preterites are used as past equivalents for the corresponding present modals in indirect speech and similar clauses requiring the rules of sequence of tenses to be applied. For example, in 1960, it might have been said that People think that we will all be driving hovercars by the year 2000, whereas at a later date it might be reported that In 1960, people thought we would all be driving hovercars by the year 2000. This "future-in-the-past" (also known as the past prospective, see: prospective) usage of would can also occur in independent sentences: I moved to Green Gables in 1930; I would live there for the next ten years. In many cases, in order to give modals past reference, they are used together with a "perfect infinitive", namely the auxiliary have and