Modals: obligation, necessity & degrees of certainty

Modals: Obligation & Necessity

Use

Expressing obligation or necessity.

Expressing lack of obligation or necessity.

Expressing past obligation.

Expressing lack of past obligation.

Modal

Must/have to/need to

Needn't/don't have to/don't need to

Had to

Needn't (+perfect inf.)/didn't have to/didn't need to

Example

I must/have to/need to go to the dentist soon.

You needn't/don't have to/don't need to work so hard.

I had to do my homework soon.

I needn't (+perfect inf.)/didn't have to/didn't need to wake up early

Use

Expressing certainty about now or generally.

Expressing certainty about the past.

Expressing probability about now, the future or generally.

Expressing probability about the past.

Expressing possibility about now, the future or generally.

Expressing possibility about the real past.

Expressing possibility about a hypothetical past.

Modal

Must/can't/couldn't

Must/can't/couldn't(+ perfect infinitive)

Should/ought to

Should/ought to(+perfect infinitive)

Could/may/might

Could/may/might(+perfect infinitive)

Could/might(+perfect infinitive)

Example

That must be the book I was expecting

You must have been working really hard.

You ought to/should do your homework soon.

This problem ought to/should have solved days ago.

This could/may/might be problematic.

That could/may/might have been a problem.

You could/might have talked to me before.