We use modal auxiliary verbs can, may, must in the English language for various meanings - ability, possiblity, probability, certainty, permission, prohibition, obligation, opinion, speculation, etc.
1. It is used to express the ability to do something.
I can swim very well.
Can he speak English fluently? - No, he can't.
We cannot sing at all!
2. It expresses the possibility to do something.
We can go to the seaside at last. Our holidays start next week.
3. We use it to say that something is probable.
It can be John. He has blond hair and he is wearing glasses.
4. It expresses the permission to do something.
Why not? You can marry her. She is a nice girl.
1. It is used for permissions.
You may borrow my car. I won't need it.
May I smoke here? - No, you can't, I'm sorry.
2. It is used to express probability or prediction.
They may call tomorrow. I hope so.
The main difference between may and can is in style. May is more formal than can. Can is typical of spoken English.
3. The opposite of may is must not or may not.
May I smoke here?
- You mustn't smoke here. (strong prohibition)
- You may not smoke here. (more polite, very formal)
- You can't smoke here. (informal spoken English)
Must - Need not
1. It is used for strong obligations. It is personal, because it expresses the speaker's opinion or will.
I must clean my teeth. I want to be healthy.
You must go there. And do it right now!
2. It means a strong recommendation.
You must see it. It's the best film I've ever seen.
3. We use it to show the certainty of the speaker.
They must be at school by now. It's already 9 o'clock.
4. The opposite of must is need not.
Mum, must I wash up? - No, you needn't. I've already done it.
Must not has a different meaning. It is used to express prohibition that involves the speaker's will.
We mustn't come late today. Or the teacher will be very angry.
He mustn't enter this room. It is dangerous.
Normally, these modal auxiliary verbs are not used in different tenses. The past tense of can is could, may and must, however, only have the present form. All the other tenses must be formed in a different way.
We make the passive voice with a verb + be + past participle: This can be done. The laws must be respected.