Modal verbs

Modal verbs (can/could, may/might, must, need, shall/should, ought to, will/would, be to, have to) do not name any actions made by a person, but denote the person’s attitude towards the surrounding world. They are used with great frequency and with the wide range of meanings. They express such ideas as willingness and ability, permission and refusal, obligation and prohibition, necessity, promise and intention; almost all modal verbs can express degrees of certainty, probability, or possibility.

They have certain characteristics in common:

1. They need no auxiliaries to form questions and negations;

2. The verbs can, may, shall, will have two tense forms of the indicative mood – the present and the past.

The present tense	The past tense
can	could
may	might
shall	should
will	would

3. The forms could, might, should, would instead of denoting a past action may denote an unreal action and in this case they have no meaning of a past action; they are used with the indefinite infinitive when we speak of the present or the future, and with the perfect infinitive when we speak about the past.

4. The verbs must and need have only one form of the indicative mood. They are used with the indefinite infinitive when we speak of the present or the future, and with the perfect infinitive when we speak about the past.

5. The modal verb ought has only one form. The form of the infinitive after the verb ought shows whether we are speaking of the present-future or of the past.

6. Modal verbs are always used in combination with the infinitive.

Unlike all other verbs, modal verbs do not denote an action but are used to express abstract modal meanings of possibility, necessity, volition, etc. Sometimes they are also used to express different emotions such as doubt, surprise, reproach, etc.

As far as their function in the sentence is concerned, modal verbs are structural. Due to the fact that they express modal relations, they are never used as independent parts of a sentence. They are always used in combination with the infinitive making up part of the compound modal predicate.

The infinitive associated with a modal verb may be used in any form, i.e. non-perfect or perfect, non-continuous or continuous, active or passive.

Can / could

The verb “can” has two forms: “can” for the Present Tense and “could” for the Past Tense; the expression “be able to” which has the same meaning can be used to supply the missing forms of the verb “can”. (e.g. I had not been able to imagine how weakened she was.)

The verb “can” is used to express:

Meaning	Forms of the modal verb	Other ways to express the same meaning	Sentence patterns
Mental, physical and circumstantial ability (all types of sentences)	Can / can’t, could / couldn’t	To be able to; to know how to do something; to have the ability to do something	Can you lift this heavy chair? I couldn’t swim quite well at the age of 8. He can see nothing without his glasses.
Permission (in questions and statements)	Can – present tense; could – in reported speech	To permit; to be allowed	Can we keep it? You can go now.
Request (in questions)	Can; could (more polite)	To permit; to be allowed	Could you do it for me?
Prohibition (negative sentences)	Can’t	Not to permit; not to be allowed	You can’t cross the street here.
Unreality (all types of sentences)	Could do – present; could have done – past	Would be able to do something; would have been able to do something	But for this I could do it quite easily now.I could have got here earlier if I had known that you were waiting for me.
Astonishment, strong doubt about the present, about the past (in questions)	Can somebody do? Could somebody do? Can / could somebody have done?	Is it possible that..? Do you believe that ..?	Can he be her husband? He is twice as old as she is.Can he have made it?
Incredulity, improbability about the present (in negative sentences)	Somebody can’t do … somebody couldn’t do…	It’s hardly possible that; I refuse to believe that …	She can’t say such words.
Improbability about the past ( in negative sentences)	Somebody can’t have done; somebody couldn’t have done	I don’t believe that … It is impossible that… It’s incredible that … I don’t think it’s possible that …	He couldn’t have made such a mistake.
The weakest probability	Can / could	It’s possible; Perhaps; By chance	You could be right, I’m not sure.
Emotional colouring (in special questions)	Can / could	—	What can you know of such things? What can they be speaking about? How can you have made such a mistake?

Note: to distinguish between a question about somebody’s ability to do something and the expression of strong doubt we use the Indefinite infinitive in the first case and the Continuous infinitive in the second (with the exception of the verbs to be, to know, to love, to hate and others).

Compare: Can he write poetry?

Can he be writing poetry?