Can, may, must
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.
CAN
1. It is used to express the ability to do something.
I can swim very well.
Can he can 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.
MAY
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.
MUST
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.
Should
We use should and shouldn't to give advice or to talk about what we think is right or wrong.
You should means something like I think it is a good idea for you to do it.
You shouldn't means something like I think it is a bad idea for you to do it. Would I wouldto go to the beach.
Me gustaría ir a la playa
2. Would you like a beer?
¿Le apetecería una cerveza?
Espero te ayude
