PAST MODALS!

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

I can drive. I could drive when I was 16. I have to go to California. I had to go to California. Lenny will pay tomorrow. Lenny said he would pay tomorrow.

(present ability) (past ability) (present obligation) (past obligation) (future intention) (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.