They are used in order to indicate modality. They allow speakers to express certainty, possibility, willingness, obligation, necessity, ability. Here are some characteristics of modal verbs:
- They never change their form. You can't add "s", "ed", "ing"...
He must works in the garden
He must work in the garden
- They are always followed by an infinitive without "to" (e.i. the bare infinitive.)
They could to go to the beach
They could go to the beach
Below, a list of the modals of possibility, obligation and prohibition.
|Possibility||may||It shows possibility in the future:
I may go to the cinema later.
|might||It shows possibility in the future. It is less formal than “may”:
Your keys might be in the car.
|could||It shows possibility in the future:
If we don't hurry we could be late.
|can||It is used to make general statements about what is possible:
It can be very hot in summer.
|Obligation||have to||It shows that the obligation comes from somebody else. It’s a law or a rule and the speaker can’t change it:
Children have to go to school.
|must||It shows that the obligation comes from the speaker. It isn’t a law or a rule:
You must study for that exam.
|Prohibition||can't||It shows that something is not permitted. It is against the rules:
You can't park your car in front of that door.
|mustn't||It is usually used when the prohibition comes from the person who is speaking:
You mustn't speak when the teacher is speaking.