Use: We use modals to express possibility, certainty, necessity, and conditionality; to give advice and permission; and to request and warn.

Modals: can, could, would, should, might, may, must, will, shall, have to, had better, and ought to.

Functions Modals Examples
possibility must, will/would, should, may, can/could, might I might try riding a unicycle.
ability can, could I can ride a unicycle.
necessity must, have to, should I must try riding a unicycle while I'm here.
permission may, could, can Can I try your unicycle?

You should make more of an effort in your studies. (advice)
You should have made more of an effort in your studies. (past)

He said that he might come if he can get out of work early. (possibility)
He said that he might have come if he could have gotten out of work early. (past)

Jerry would go to Hawaii if he had some vacation time. (conditional)
Jerry would have gone to Hawaii if he had had some vacation time. (past)

Susan had better start arriving on time if she wants to keep her job. (warn)
Susan needs to work harder if she wants to pass the course. (necessity)

If he missed the meeting, he must have been very sick. (certainty; past)

Rules - Ex.1 - Ex.2 - Rules - Ex.3