MODALS IN PAST

 Past modals with have

EXPLANATION


http://learnenglish.britishcouncil.org/en/grammar-reference/modals-%E2%80%93-deduction-pasthttp://www2.cambridge.org/interchangearcade/launch.do?activityID=521&level=2&unit=15&type=Cup_sentence_ordering&stage=2
http://www.brays-ingles.com/gramatica-ingles-modals.html

Using modal verbs to talk about the past:

must / might / could / may / can't + have + past participle

• must have + past participle
• might / might not have + past participle
• could / couldn’t have + past participle
• may / may not have + past participle
• can’t have + past participle

For example:

You: Where was Julie last night?
David:

• She must have forgotten about our date.
• She might have worked late.
• She could have taken the wrong bus.
• She may have felt ill.
• She can’t have stayed at home.

Some past modals can be formed by using have + the past participle of the main verb immediately after the modal. (should have, could have, would have, etc.)
However, since modals express possibility, intention, obligation, etc., they do not always indicate a definite tense. Therefore, when using past modals with have, special meanings need to be considered.

I should go to the funeral. I should have gone to the funeral. Lex might take Karen to the airport. Lex might have taken Karen to the airport. Lex could have taken Karen to the airport. Lex would have taken Karen to the airport. Otis didn't come to work yesterday. He had to take care of his children. His children must have been sick.

(I feel an obligation to go--later.) (I didn't go. Now I regret it.) (It's a future possibility.) (He may be on his way there now.) (Most likely he didn't.) (He didn't. He had an excuse.) (past fact) (past obligation) (conjecture about the past)