Modal verbs express the speaker’s judgment about ability, permission, possibility, necessity, obligation, advice, requests, and deduction. They form a special auxiliary class that combines with a bare infinitive to show attitude toward the main action rather than time alone. For related auxiliary behavior, compare Auxiliary Verbs.

Modal verbs have a fixed form and do not take agreement endings or participles. After a modal, the main verb appears as a bare infinitive, and negation is formed with not. Questions are formed by inversion, with the modal placed before the subject.

Idea	Example
Modal + bare infinitive	She can swim.
Modal + not	He should not leave.
Inversion for questions	Can you help?

Can and could commonly express ability, while may and can often express permission. Could is also the more polite or tentative past or contrastive form of can. For broader question forms, compare Asking Questions.

Idea	Example
Ability in the present	She can solve the puzzle.
Ability in the past	He could read very early.
Permission	You may enter now.

May, might, could, and will all help locate an event on a scale from possible to expected. May and might usually present possibility, while will can signal a strong expectation about the future. Shall is now rare in American English and survives mainly in formal British English offers and suggestions.

Idea	Example
Possible event	It may rain later.
Uncertain event	They might arrive soon.
Strong expectation	She will know the answer.

Must and have to both express necessity, but must often feels like the speaker is imposing the obligation, while have to often reflects outside rules or circumstances. Should and ought to express advice, recommendation, or mild obligation. Need to and be able to also function as semi modal and periphrastic alternatives when a full modal form is unavailable or less natural.

Idea	Example
Strong obligation	You must stop here.
External necessity	We have to wear badges.
Advice	You should rest today.

Would, could, and may are common in polite requests because they soften the force of the question. Must is much stronger and usually sounds like a rule or insistence rather than a request. In careful style, shall can appear in formal British offers or suggestions.

Idea	Example
Polite request with would	Would you open the window?
Polite request with could	Could you wait a moment?
Formal offer with shall	Shall I call a taxi?

Modal perfect forms combine a modal with have and a past participle to talk about earlier possibility, necessity, regret, or inference. This pattern is especially useful for judgments about past events and is closely linked to Past Participles and Present Perfect.

Idea	Example
Past possibility	She might have missed the train.
Past deduction	He must have forgotten the key.
Past criticism	You should have called earlier.

Would, could, and might are common in conditional clauses, especially in second and third conditionals. These modals help show imagined results, unreal possibilities, and outcomes tied to past unreal conditions. For the broader pattern, see Conditional.

Idea	Example
Unreal present result	If I had time, I would travel.
Unreal past result	If she had studied, she could have passed.
Limited possibility	If you asked, they might help.

In indirect speech, modal verbs often backshift when the reporting verb is in the past. Will commonly becomes would, and can commonly becomes could, while some modals such as must may stay the same depending on meaning. For the larger pattern of shifted speech, see Indirect Speech.

Idea	Example
Will becomes would	He said he would come.
Can becomes could	She said she could wait.
Must may stay the same	They said we must leave now.

May and might both express possibility, but might is usually weaker, more tentative, or more remote. Must and have to overlap in meaning, yet must often feels internal or speaker imposed, while have to often reflects an external requirement. Modals do not have full tense systems, so English uses periphrastic forms such as could, would, and have to when a regular past form is needed.

Idea	Example
May and might	It may happen, but it might not.
Must and have to	I must finish now, but I have to leave at six.
No full tense form	She could not attend yesterday.

Can, could, may, might, will, would, shall, should, must, and ought to are the high frequency forms learners meet earliest. Together they cover the main English meanings of ability, permission, possibility, probability, obligation, advice, and requests. Mastery depends on recognizing the meaning each modal adds to the bare infinitive that follows it.