Modal verbs are used to express ideas like ability, permission, obligation, and possibility. The main modal verbs discussed are: - Can expresses ability or permission. It is also used for possibility or impossibility. - Could expresses ability in the past or possibility. It also makes polite requests. - Have to expresses obligation in the affirmative and interrogative but lack of obligation in the negative. - May expresses possibility or permission, especially in formal contexts. - Might expresses small possibility. - Must expresses present or future obligation. Mustn't expresses prohibition. - Should gives advice.