Modals and their meanings

can

General ability (present)

Informal requests or permission

could

General ability (past)

Polite, informal requests or suggestions

may

Formal requests or permission

Possibility

might

Possibility

must

Necessity (present or future)

Strong possibility or near certainty (present or past)

should

Suggestions or advice

Obligations or duties

Expectations

will*

Certainty

Requests

Promises and offers

would

Polite requests

Habitual or repeated actions (past)

* Shall traditionally was the modal used with I or we in place of will. It is now used mainly for polite requests or in very formal contexts (Shall I call you soon?).