Modal verbs

Can, could, may, might, must, shall, should, will and would are the modal verbs.

They always go together with the infinitive of the main verb, for example:

I must go.

You should call him.

John can meet me at the airport.

What is the difference between modal verbs and other verbs?

The modal verbs are different from other verbs:

My daughter is four years old and she can read and write.

Mr Johnson might come tomorrow.

Can you close the window, please? BUT

Does he always close this window?

I must go to the dentist. BUT

I had to go to the dentist yesterday.

We may sit here. BUT

I want to sit here.

they don't have infinitive forms, they can't be used after "to" or another modal verb

A candidate should be able to list the pros and cons of functional programming. WRONG A candidate should can...

Verb	Negative form	Meaning	Examples
can	cannot (can't)	abilities, physical or intellectual a possibility to do something	I can speak German. You can run fast. I can help you. He can call a cab.
must	mustn't	must — need to mustn't — wrong, not acceptable	I must hurry or I'll be late. Son, you mustn't behave like this.
should	shouldn't	recommendation, it is a good thing to I think ... should / I don't think ... should	It's an interesting video. You should watch it. I don't think you should care about it.
may/might	may not/might not	be allowed, have permission something is probable but not completely sure	You may not speak loudly in a library. Dan might not come tomorrow. He says he might have some other plans.
have to	not have to	have to — obligation, necessity not have to — no need	We didn't have money for the taxi so we had to walk home. It is the same contract as last year. You don't have to read it again.

We can use modal verbs when we want to make a proposition about something. We choose the verb depending on how certain we are.

Verb	Meaning	Example
can't	I'm sure it is not true	It can't be true. I don't believe it.
could	possibly, maybe	She could be on the train to Paris now.
might	possibly, maybe	I might join you later.
must	I'm sure it is true	It must be a mistake!
will	it is certain	She will be at home by now.

When we express probability with modal verbs, we can use more constructions than just "a modal verb + infinitive".

- modal verb + be + -ing

In fact, it is the infinitive of the present continuous form (to be joking, to be leaving, to be talking).

Examples

She might be having lunch.

This can't be happening again!

You must be joking.

- modal verb + have + past participle

When we make a guess or express regrets about the past situation, we use a past modal verb.

Examples

He must have gone out.

I should have listened to you.

Jessica didn't invite me, but I couldn't have come anyway.