Modals in the Past

Past modals
come in two forms. The first type is the easiest and usually requires only a simple word change:

I can drive. (preswent ability)                              

I could drive when I was 16. (past ability)

I have to go to California. (present obligation)   

I had to go to California. (past obligation)

Lenny will pay tomorrow. (future intention)     

Lenny said he would pay tomorrow. (future reported from the past)

Past modals with have
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 feel an obligation to go--later.)
I should have gone to the funeral. (I didn't go. Now I regret it.)

Lex might take Karen to the airport. (It's a future possibility.)
Lex might have taken Karen to the airport. (He may be on his way there now.)
Lex could have taken Karen to the airport. (Most likely he didn't.)
Lex would have taken Karen to the airport. (He didn't. He had an excuse.)

Otis didn't come to work yesterday. (past fact)
He had to take care of his children. (past obligation)

His children must have been sick. (conjecture about the past)

	Modals in the Present	Modals in the Past
Obligation	You must / have to stop when the traffic lights are red.	You had to stop.
Advice	You should see a doctor.	You should have seen a doctor
Prohibition	You mustn't smoke here.	You mustn't have smoked there.
Ability	I can run fast.	I could run fast. now I am old.
Certainty	He has a Rolls Royce. He must be very rich. He can't be American. His English is terrible.	He must have been rich. He had a big house and an expensive car. He can't have written that poem. He was illiterate.
Permission	Can I go out?	She could drive her father's car when she was only 15.
Possibility	It may / can / could / might rain. It's cloudy.	I guess it may / can / could / might have been Lacy on the phone.
Lack of necessity	You don't have to / needn't buy any tomatoes. There are plenty in the fridge.	You didn't have to / didn't need to buy tomatoes.

taken from:

http://www.eslgold.com/grammar/past_modals.html

http://www.myenglishpages.com/site_php_files/grammar-lesson-modals-present-past.php