1. polite request

2. formal permission

You may leave the room.

3. Less than 50% certainty

--Where's Jon?

He may be at the library

He may have been at the library

1. less than 50% certainty

--where's John?

He might be at the library

He Might have been at the library

2. polite request (rare)

1. advisability

I should have studied last night.

2. 90% certainty

She should do well on the test (future only, not present)

She should have done well on the test.

1. advisability

2. 90% certainty

She ought to do well on the test. (future only, not present)

She ought to have studied last night.

1. advisability with threat of bad result

You had better be on time, or we will leave without you.

(past form uncomon)

1. expectation

Class is supposed to begin at 10.

Class was supposed to begin at 10.

1. strong expectation

You are to be here at 9:00.

You were to be here at 9:00

1. strong necessity

I must go to class today

I had to go to class yesterday

2. prohibition (negative)

You must not open that door.

3. 95% certainty

Mary isn't in class. She must be sick (present only)

Mary must have been sick yesterday.

1. necessity

I have to go to class today.

I had to go to class yesterday

2. lack of necessity (negative)

I don't have to go to class today.

I had to go to class yesterday.

1. necessity

I have got to go to class today.

I had to go to class yesterday.

1. 100% certainty

He will be here at 6:00 ((future only)

2. willingness

--the phone's ringing.

I'll get it.

3. polite request

1. 100% certainty

He is going to be here at 6:00 (future only)

2. definite plan

I was going to paint my room, but I didn't have time.

1. ability / possibility

I can run fast.

I could run fast when I was a child but now I can't.

2. informal permission

You can use my car tomorrow.

3. informal polite request

4. impossibility (negative only)

That can't be true!

That can't have been true!

1. past ability

I could run fast when I was a child.

2. polite request

Could you help me/

3. suggestion

--I need help in math.

You could talk to your teacher.

You could have talked to your teacher.

4. less than 50% certainty

--Where's John?

He could be at home.

He could have been at home.

5. impossibility (negative only).

That couldn't be true!

That couldn't have been true!

1. ability

I was able to help him.

1. polite request

2. preference

I would rather go to the park than stay home.

I would rather have gone to the park.

3. repeated action in the past

When I was a child, I would visit my grandparents ever weekend

1. repeated action in the past

When I was a child, I would visit my grandparents every weekend.

1. polite question to make a suggestions

2. future with "I" or "we" as subject