Modal auxiliaries 2: past

Had to is the past form of must and refers to a past obligation.

Sorry I'm late, I had to post some letters.

The negative form is didn't have to.

Must have refers to past certainty, (see below)

Where * should appears, ought to is also possible.

1. Expectation

Should have refers to something which was supposed to happen.

The parcel I sent you* should have arrived by now.

2. Criticism of an action

You * shouldn't have eaten so much last night.

3. Should have and verbs of thinking

The past form knew in the example is an unreal tense, and the should have form is used according to 'sequence of tenses'. I should have thought you knew.

4. With be and adjectives describing chance

It was strange that you should have been staying in the same hotel last уear.

5. As a polite expression of thanks on receiving a gift or a favour

- I've done the washing up for you. - Oh, you really shouldn't have!

Could have

1. Could have refers to past possibility or uncertainty.

David could have won the race if he had tried. (possibility/ability)

It could have been Sue, I suppose. (uncertainty)

2. Couldn't have is also possible for both meanings.

3. Couldn't have can be used with comparative adjectives.

We couldn't have been happier in those days.

4. Could have can also express unwillingness.

We couldn't have left the dog on its own. (so we didn't)

Could

1. Could refers to past permission or past ability.

When I was sixteen I could stay out till 11.00. (I was allowed to)

Mary could swim when she was three. (she actually did)

2. Compare:

Mary could have swum when she was three. (but she didn't)

1. Might have refers to past possibility which did not happen.

You might have drowned!

2. Might have and may have refer to uncertainty.

I suppose I may have been rather critical.

3. Both can be used in the negative to express uncertainty.

They might not have received our letter yet.

4. Might have is used to express annoyance at someone's failureto dosomething. There is strong stress on the words underlined.

You might have told me my trousers were split!

5. I might have known + would is an idiom by which the speaker expresses ironically that an action was typical of someone else.

I might have known that he would be late.

1. These refer to the speaker's certainty about a pastaction.

Someone must have taken it. (I am sure they did.)

You can't have lost it. (I am sure you didn't.)

2. Both can also be used with surely in exclamations.

Surely you can't have eaten all оf it!

1. Needn't have done refers to an unnecessary action which was actually done.

You needn't have paid all at once. (You did pay.)

2. Didn't need to refers to an unnecessary action whichwas not done.

I didn't need to go to the dentist again, luckily.

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