• The modal auxiliaries are:
can, could may, might shall, should will, would must
• Modal auxiliaries (or “modals”) are a type of auxiliary verb. Because they are auxiliaries the modals cannot stand by themselves as the head of a verb phrase as a “main” (or “full”) verb can.)
• There is important contrast between the modals and the other type of auxiliary, the primary verbs be, do, and have. Auxiliaries of both types are function words and therefore have grammatical meaning. Primary verbs, however (when they are being used as auxiliaries) have only grammatical meaning whereas modal auxiliaries also have lexical meaning. The study of the precise meanings of the modal auxiliaries—and they all have more than one—is a large subject. Generally speaking, we can say that modals make a contribution to the meaning of the main verbs they accompany by indicating the degree of obligation or necessity attached to action referred to by the main verb. For example the difference between the sentences, “Jack talks to Jane every day” and “Jack should talk to Jane every day” is that the second, unlike the first, indicates the opinion of the speaker—that Jack is under some degree of obligation to speak to Jane every day. In other words, it would be morally wrong, or foolish, or rude for Jack not to talk to Jane every day.
• Modals are always placed at the beginning of the verb phrase and are are always followed by a “base form” (of the main verb or another auxiliary). For example:
Jane must tell Dick the truth • Jane should be here by now • Jane could answer that question
• In addition to the modal auxiliaries, there is a large and important group of “marginal modals.” These items are like the modal auxiliaries in that they also add “modal meaning” to a verb phrase; but they are unlike them, first, in that most of them are followed by a to-infinitive instead of a base form and, second, in that several of them can be used as main verb. Some of the most important marginal modals are:
need to • have to • have got to • be bound to • be able to • be allowed to • ought to • be supposed to • be likely to
had better • be willing to • would sooner • be liable to • happen to
• teaching note: a common and persistent error among ESL students is the use of the to-infinitive instead of the base form as in Jane must to tell Dick the truth. This error is perhaps encouraged by the fact that most of the marginal modals are followed by a to-infinitive.