- Modal auxiliary verbs are can, could, may, might, must, will, would, shall, should, ought, dare and need.
Modal verbs have no -s in the third person singular.
She might know his address. (NOT She mights . . .)
Questions and negatives are made without do.
Can you swim? You shouldn’t
After modal auxiliary verbs, we use the infinitive without to. (Ought is an exception:)
I must remember to write to Leslie.
We do not use modal verbs for situations that definitely exist, or for things that have definitely happened. We use them, for example, to talk about things which we expect, which are possible, which we think are necessary, which we are not sure about, or which did not happen.
- She will be here tomorrow.
I may come tomorrow if I have time.
She could be in London or Paris or Tokyo — nobody knows.
You must come and have dinner with us some time.
What would you do if you had a free year?
She should have seen a doctor when she first felt ill.
We use the structure [modal verb + perfect infinitive] (for example
must have seen, should have said) to talk about the past. This structure is used for speculating (thinking about what possibly happened) or imagining (thinking about how things could have been different).
- [modal verb + have + past participle]
She’s two hours late. What can have happened?
You could have told me you were coming.
The potatoes would have been better with more salt.
The plant’s dead. You should have given it more water.