One of the problems with writing about modal verbs is that they are classified and analysed differently by different authorities. This particular description recognises the influence of Michael Lewis' book The English Verb.
A modal verb, also known as a modal auxiliary verb, is a verb we can use with another verb to express possibility or necessity along with concepts such as obligation, permission, inevitability etc. They are used to express the speaker's opinion of a subject and thus to affect the mode of a sentence.
The modal verbs in English are:
- They can be negated. In other words we can say "will not" but we cannot say "run not"—at least in modern English.
- They have weak forms, especially in the negative.
- They cannot co-occur in sentences. We cannot say, "This may will happen".
- They can be used to form polar questions. (Yes/no questions).
- They can be used in ellipsis. In answer to the question "Can you swim?" the answer "Yes, I can" is understood to mean "Yes, I can swim."
- They are not conjugated or changed in any way. That means they have no third person "s", no past form and no "ing" form.
- They are followed by the bare infinitive of another verb. That means they are not followed by "to".
Present, past and future
The relationship of the pure models to time is complicated and not all authorities agree on the best way to express it.
See full article The future in English
The modal "will" is often taught as "the future" but it is, in fact, only one way of referring to the future in English along with the present simple, the continuous forms, with "going to" etc. Furthermore it is not the only modal verb which can refer to the future.
Contrast the phrase "I will visit him", "I might visit him", and "I should visit him". The first expresses a firm intention (or perhaps more likely a decision made at the moment of speaking) the second indicates a possibility and the third implies a personal obligation.
Furthermore "will" could refer to the present. Somebody knocks on the door and you say: "Ah that will be Bob, he said he would call round." We consider that it is inevitably Bob because of the logic of the situation. Consequently "will" indicates inevitability rather than futurity.
Some maintain that "could" is the past of "can", that "would" is the past of "will" and so on. While this may sometimes be true in the case of reported speech it is by no means always the case as all modals can be—and usually are—used with a present sense. The only possible exception being "could" which frequently has a past meaning.
To complicate the issue there There are also some verbs which can have some, but not all, the properties of modal verbs. These are:
- ought (to)
- had ('d) better
- used to
- going to
- The New Oxford Dictionary of English