He is definitely not 18 yet.
He can't be 18 yet.
I strongly recommend you to stop smoking.
You ought to stop smoking.
It's better not to trust her.
You shouldn't trust her.
It's very important to find my keys.
I must find my keys.
It's possible that Jane will visit the USA next year.
Jane may/might/could visit the USA next year.
I'm sure that Mrs Smith doesn't have grandchildren yet.
Mrs Smith can't have grandchildren yet.
Let me help you.
Shall I help you?
I'm sure she is at home because the lights are on.
She must be at home because the lights are on.
It's not a good idea to eat so many sweets.
You shouldn't eat so many sweets.
I'm thinking about taking Spanish lessons.
I may/might/could take Spanish lessons.
They don't allow talking on the phone in hospital.
You mustn't talk on the phone in hospital.
I know that isn't Janet because she is in Egypt now.
It can't be Janet because she is in Egypt now.
It isn't necessary to cook while you are here.
You needn't/don't have to cook while you are here.
He knows how to swim.
He can swim.
She didn't manage to beat the record.
She wasn't able to beat the record.
It isn't necessary to make your bed when you stay at the hotel.
You don't have to /needn't make your bed when you stay at the hotel.
Do you mind if I sit next to you?
Can/could/may I sit next to you?
Why don't we go to a disco club this evening?
Shall we go to a disco club this evening?