Types of modals

Modals include modal verbs, semi-modal verbs (also called marginal modals) and other modal expressions. They combine with main verbs and modify their meanings. A modal may have several different meanings, while similar meanings may be expressed by using different modals:

He can't be at home; I've just met him. (deduction)
Unless you finish your homework, you can't go to the cinema. (prohibition)

Can I help you? (offer)
May I help you? (offer)

Modal verbs

The modal verbs (or modal auxiliary verbs) are: can, could, may, might, will, shall, would, should and must.

• Modal verbs always come first in a verb phrase and are followed by a bare infinitive. When used with a perfect infinitive, modal verbs usually refer to past time:
  

  I could hear the dog barking outside. (modal + simple bare infinitive)
  You must be joking. (modal + continuous bare infinitive)
  He may have caught the train. (modal + perfect bare infinitive)
  You must have been waiting for hours. (modal + perfect continuous bare infinitive)

  Contracted forms of will and would are often used in spoken and in informal written language ('ll and 'd):

  I'd tell you if I knew.
  They'll be here soon.

• Modal verbs take no -s in the third person singular:
  

  He might be at the office.

• Modal verbs form their negative and interrogative like other auxiliaries and not with do:
  

  I can't swim.
  Can you swim?

  The following contracted negative forms are often used in spoken and in informal written language:

  cannot » can't
  could not » couldn't
  might not » mightn't
  will not » won't
  shall not » shan't
  would not » wouldn't
  should not » shouldn't
  must not » mustn't

• Modal verbs have no proper past tense; however, could, would, might and should may be used to refer to past time:
  

  I could swim when I was five.

• Modal verbs have no infinitive, -ing or past participle forms and cannot be followed by other modal verbs. When necessary, modal idioms or other expressions are used instead of them:
  

  If you want to be a sailor, you must can swim.
  If you want to be a sailor, you must be able to swim.

  I have canned swim since the age of five.
  I have been able to swim since the age of five.

Semi-modal verbs

The semi-modal verbs (or marginal modals) are: dare, need, used to and ought to. They behave similarly to modal verbs but also share some characteristics with main verbs:

How dare she criticise us? (as a modal verb, the interrogative formed without do)
He didn't dare to look back. (as a main verb, followed by a to-infinitive and the negative formed with do)

Need you make so much noise? (as a modal verb, the interrogative formed without do)
You needn't have been so rude. (as a modal verb, the perfect infinitive used to refer to past time)
Do you need to use the hairdryer? (as a main verb, followed by a to-infinitive and the interrogative formed with do)

They used to live by the sea. (unlike a modal verb, followed by a to-infinitive)

You ought to know that by now. (unlike a modal verb, followed by a to-infinitive)

Other modal expressions

Besides modal verbs and semi-modal verbs, there are other expressions which can express modal meanings. Some of these are formed with be:

be able to
be allowed to
be about to
be bound to
be going to
be likely to
be obliged to
be supposed to
etc.

Other expressions that carry modal meanings are: be to, had better, have (got) to, would rather.

Modal verbs, semi-modal verbs and other modal expressions are discussed in detail in the following chapters.