: 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, needs 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)

Modals: must, have to

Rule 1: We use MUST to express personal feelings about having to do something.
Example 1: I must call Uncle John this weekend to see how he's doing.
Example 2: Republicans must give up something if there is ever going to be an agreement on debt reduction.

Rule 2: We use HAVE TO to express facts.
Example 1: Unfortunately, I'm going to miss the party. I have to finish this report before tomorrow.
Example 2: He has to wear safety glasses while welding.

Rule 3: We use MUST to talk about present and future actions.
Example: They must get organized if they want this business to succeed.

Negative Forms
Rule 4: MUST NOT and DO NOT HAVE TO have different meanings.
Example 1: He must not take this medication before driving. = For his safety, he cannot take this medication before driving.
Example 2: He doesn't have to take the early train. Let's make it for eleven in the morning. = It is not necessary to leave that early because the meeting will be later in the morning.