THE MODAL VERBS (GENERAL CONCEPTS ON MODAL VERBS)

Here's a list of the modal verbs in English:

can	could	may	might	will
would	must	shall	should	ought to

Modals are different from normal verbs:
1: They don't use an 's' for the third person singular.
2: They make questions by inversion ('she can go' becomes 'can she go?').
3: They are followed directly by the infinitive of another verb (without 'to').

Probability:

First, they can be used when we want to say how sure we are that something happened / is happening / will happen. We often call these 'modals of deduction' or 'speculation' or 'certainty' or 'probability'.
For example:

-It's snowing, so it must be very cold outside.

-I don't know where John is. He could have missed the train.

-This bill can't be right. £200 for two cups of coffee!

Ability:

We use 'can' and 'could' to talk about a skill or ability.
For example:

-She can speak six languages.

-My grandfather could play golf very well.

-I can't drive.

Obligation and Advice:

We can use verbs such as 'must' or 'should' to say when something is necessary or unnecessary, or to give advice.
For example:

-Children must do their homework.

-We have to wear a uniform at work.

-You should stop smoking.

Permission:

We can use verbs such as 'can', 'could' and 'may' to ask for and give permission. We also use modal verbs to say something is not allowed.
For example:

-Could I leave early today, please?

-You may not use the car tonight.

-Can we swim in the lake?

Habits:

We can use 'will' and 'would' to talk about habits or things we usually do, or did in the past.
For example:

-When I lived in Italy, we would often eat in the restaurant next to my flat.

-John will always be late!

Past modals:

The past modals 'could have + past participle', 'should have + past participle' and 'would have + past participle' can be confusing.