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
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
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.
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. We could watch a movie. B. Shall we watch a movie?
Both are suggestions
Correct: This guy mustn’t be a thief, I’m sure he's not guilty!
mustn't -> can't
Correct: You mustn’t have read the whole book during the night! It’s huge!
mustn't -> can't
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.
Hai can have done his homework - but I think it's unlikely.
can -> could/ may/ might
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
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.
Correct: I couldn’t have talked during the lessons. Now I don’t understand anything and will fail the test.
Couldn't -> Shouldn't
Correct: It mustn’t be true, it can be a lie.
It can't be true, it must be a lie.
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