Grammar: Obligation, Necessity and Permission

There's someone at the door. It can/must be the postman.

• Can

• Must

Don't worry, you don't have to/mustn't pay now.

• Don't have to

• Mustn't

I think you had better/would better take a pullover with you.

• Had better

• Would better

Jones could/must be president if Smith has to resign.

• Could

• Must

Sorry, I can't stay any longer. I have to/might go.

• Have to

• Might

It was 5 o'clock an hour ago. Your watch can't/mustn't be right.

• Can't

• Mustn't

It's a school rule, all the pupils have to/must wear a uniform.

• Have to

• Must

I suppose that our team must/should win, but I'm not sure.

• Must

• Should

Let's tell Diana. She could/might not know.

• Could

• Might

In my opinion, the government might/should do something about this.

• Might

• Should