Modal Verb

The modal verbs are can, could, may, might, shall, should, will and would.

The modals are used to show that we believe something is certain, probable or possible:

Possibility:

We use the modals could, might and may to show that something is possible in the future, but not certain:

They might come later. (= Perhaps/Maybe they will come later.)
They may come by car. (= Perhaps/Maybe they will come by car.)
If we don’t hurry we could be late. (= Perhaps/Maybe we will be late)

We use could have, might have and may have to show that something was possible now or at some time in the past:

It’s ten o’clock. They might have arrived now.
They could have arrived hours ago.

We use the modal can to make general statements about what is possible:

It can be very cold in winter. (= It is sometimes very cold in winter)
You can easily lose your way in the dark. (= People often lose their way in the dark)

We use the modal could as the past tense of can:

It could be very cold in winter. (= Sometimes it was very cold in winter.)
You could lose your way in the dark. (= People often lost their way in the dark)

Impossibility:

We use the negative can’t or cannot to show that something is impossible:

That can’t be true.
You cannot be serious.

We use couldn’t/could not to talk about the past:

We knew it could not be true.
He was obviously joking. He could not be serious.

Probability:

We use the modal must to show we are sure something to be true and we have reasons for our belief:

It’s getting dark. It must be quite late.
You haven’t eaten all day. You must be hungry.

We use must have for the past:

They hadn’t eaten all day. They must have been hungry.
You look happy. You must have heard the good news.

We use the modal should to suggest that something is true or will be true in the future, and to show you have reasons for your suggestion:

Ask Miranda. She should know.
It's nearly six o'clock. They should arrive soon.

We use should have to talk about the past:

It's nearly eleven o'clock. They should have arrived by now.

Ability:

We use can to talk about someone’s skill or general abilities:

She can speak several languages.
He can swim like a fish.
They can’t dance very well.

We use can to talk about the ability to do something at a given time in the present or future:

You can make a lot of money if you are lucky.
Help. I can’t breathe.
They can run but they can’t hide.

We use could to talk about past time:

She could speak several languages.
They couldn’t dance very well.

We use could have to say that someone had the ability/opportunity to do something, but did not do it:

She could have learned Swahili, but she didn’t have time.
I could have danced all night [but didn't].

Permission:

We use can to ask for permission to do something:

Can I ask a question, please?
Can we go home now.

could is more formal and polite than can:

Could I ask a question please?
Could we go home now?

may is another more formal and polite way of asking for permission:

May I ask a question please?
May we go home now?

We use can to give permission:

You can go home now if you like.
You can borrow my pen if you like.

may is a more formal and polite way of giving permission:

You may go home now, if you like.

We use can to say that someone has permission to do something:

We can go out whenever we want.
Students can travel free.

may is a more formal and polite way of saying that someone has permission:

Students may travel free.

Instructions and requests:

We use could you and would you as polite ways of telling or asking someone to do something:

Could you take a message please?
Would you carry this for me please?
Could I have my bill please?

can and will are less polite:

Can you take a message please?
Will you carry this for me please?

Suggestions and advice:

We use should to make suggestions and give advice:

You should send an email.
We should go by train.

We use could to make suggestions:

We could meet at the weekend.
You could eat out tonight.

We use conditionals to give advice:

Dan will help you if you ask him.

Past tenses are more polite:

Dan would help you if you asked him.

Offers and invitations:

We use can I… and to make offers:

Can I help you?
Can I do that for you?

We can also use shall I …

Shall I help you with that?
Shall I call you on your mobile?

We sometime say I can...or I could ...or I’ll (I will) ...to make an offer:

I can do that for you if you like.
I can give you a lift to the station.
I’ll do that for you if you like.
I’ll give you a lift to the station.

We use would you like (to) ...for invitations:

Would you like to come round to morrow?
Would you like another drink?

We use you must or we must for a very polite invitation:

You must come round and see us.
We must meet again soon.

Obligation and necessity

We use must to say that it is necessary to do something:

You must stop at a red light.
Everyone must bring something to eat.
You can wear what you like, but you must look neat and tidy.
I’m sorry, but you mustn’t make a noise in here.

We use had to for this if we are talking about the past:

Everyone had to bring something to eat.
We could wear what we liked, but we had to look neat and tidy.

modals + have

We use a modal verb with have and the past participle:

Subject	Modal	Have	Past Participle
They	will	have	arrived	by now
You	might	have	seen	the film
Jack and Jill	would	have	been	late

We use a modal verb with have to refer back:

•      … from a point of time in the past:

We were very worried. Someone might have taken the car.

•      … from the present

It is nearly eight o’clock. They will have arrived by now.

•      …or from the future:

We won’t eat until they arrive. They might not have had supper.

•      or to refer to past time:

You should have helped her when she asked.

They might have got lost. Nobody knows where they are.