A. We could watch a movie. B. Shall we watch a movie?
Both are suggestions
Correct: You mustn’t have read the whole book during the night! It’s huge!
mustn't -> can't
A. I must work hard. B. I have to work hard. C. I may work hard.
A and B talk about obligation. C may talks about possibility.
A. You should listen to what he says. B. You ought to listen to what he says. C. You have to listen to what he says.
A and B have similar meaning, it’s a good idea, basically I’m giving advice. You may choose to follow it or not. C means that it’s your obligation
Hai can have done his homework - but I think it's unlikely.
can -> could/ may/ might
Correct: This guy mustn’t be a thief, I’m sure he's not guilty!
mustn't -> can't
Correct: It mustn’t be true, it can be a lie.
It can't be true, it must be a lie.
A. We couldn’t go to Paris last year. B. We didn’t have to go to Paris last year.
A talks about lack of ability in the past, even if we wanted to, we weren’t able to do it. B talks about lack of obligation, it wasn’t necessary for us to go.
A. You don’t have to wake up so early. B. You shouldn’t wake up so early. C. You needn’t wake up so early.
A and C have similar meaning, it’s not necessary to do it, but you can if you want to. B means that it’s a bad idea to wake up early
Correct: I can't swim when I was a child, but I can't now.
I couldn't swim when I was a child, but I can't now.
A: We could have a problem. B: My grandfather could speak two languages.
A: Hypothesising (i.e. it's possible in this situation). B: Past ability
You may be the most intelligent boy in the class, but I will beat you in the test! Q: What is the meaning of 'may'?
It is a concession. You are saying you understand he's the most intelligent, but don't like that fact.
Correct: I couldn’t have talked during the lessons. Now I don’t understand anything and will fail the test.
Couldn't -> Shouldn't
Your experience on this site will be improved by allowing cookies.