Modal verbs are special auxiliary verbs that add meanings like ability, permission, advice, obligation, possibility, or prediction to another verb. They do not usually describe an action by themselves; they modify the main verb. In English, a modal is followed by the base form of a verb, and the pair functions together as the verb phrase of the sentence.

The most common modal verbs are can, could, may, might, must, shall, should, will, and would. These modals are used to express a range of meanings, and context often decides the exact interpretation. Some modals sound more formal or more indirect than others, especially could, might, and would.

Word/Phrase	Definition	Example
can	ability or permission	I can swim.
could	past ability or polite possibility	Could you help me?
may	formal permission or possibility	You may leave now.
might	weaker possibility	It might rain later.
must	strong obligation or logical conclusion	You must wear a badge.
should	advice or expectation	You should rest.
will	future or willingness	I will call you.
would	polite request or hypothetical	I would like some tea.

A modal verb is followed by the base verb without to: modal + base verb. In negatives, not comes after the modal. In questions, the modal comes before the subject. Modals do not take -s in the third person singular and do not use do or does for questions and negatives.

Rule	Example
modal + base verb	She can drive.
modal + not + base verb	They should not worry.
modal before subject in questions	Can you stay?
no third person -s on modals	He can help.
no do or does with modals	Will she come?

Modals often show how strong a speaker feels about a situation, from weak possibility to strong certainty or obligation. Might and could are usually weaker than may, which is often weaker than must for conclusions. For advice, should is softer than must, and will is stronger than would for willingness or intention.

Can is common for ability and informal permission, while could can describe past ability or a more polite request. May is often used for formal permission and polite permission questions, though it can also express possibility. Choosing between can, could, and may mainly changes formality and indirectness.

Rule	Example
ability in the present: can	I can read French.
ability in the past: could	When I was five, I could ride a bike.
informal permission: can	You can sit here.
formal permission: may	You may begin the exam.
polite request: could or may	Could I ask a question?

Must expresses strong obligation from the speaker or strong rules, and it can also express a strong logical conclusion. Should expresses advice, recommendation, or expectation and is less forceful than must. In many real situations, have to can replace must for obligation, especially when the obligation comes from outside rules rather than the speaker.

Rule	Example
strong obligation: must	You must stop at red lights.
advice or recommendation: should	You should see a doctor.
expectation: should	The train should arrive soon.
strong conclusion: must	She must be at work now.

May, might, and could express possibility, but they usually differ in certainty and tone. Might is often the weakest and most tentative, while may can sound a bit more confident or formal. Could often suggests a possible option among others rather than a prediction.

Rule	Example
tentative possibility: might	I might go later.
possibility, often more formal: may	We may need more time.
one possible option: could	We could take the train.

Will is used for future reference, decisions at the moment of speaking, promises, and willingness. Shall is limited in modern English and is most common in questions offering or suggesting, especially in British English. Would is used for polite requests, offers, and more indirect statements about willingness.

Rule	Example
future decision or plan: will	I will email you tonight.
promise: will	I will be there on time.
offer or suggestion question: shall	Shall we start?
polite request: would	Would you open the window?
indirect willingness: would	I would help if I could.

Would and could are common for hypothetical situations and conditional meaning. Would often describes what happens in an imagined situation, while could often shows hypothetical ability or possibility. In conditional sentences, would and could frequently appear in the result clause to show that the situation is not certain or is not real now.

Modals have limited past forms, so English often uses modal + have + past participle to talk about past possibility, past certainty, or past obligation. This structure is used to comment on past events, often with meanings like inference, regret, or criticism. Not all modals use perfect forms equally, but could have, might have, should have, would have, and must have are common.

Rule	Example
past possibility: might have + past participle	She might have missed the bus.
past possibility or ability: could have + past participle	I could have called you.
past advice or criticism: should have + past participle	You should have told me.
past inference: must have + past participle	They must have left early.
hypothetical past result: would have + past participle	I would have gone if I had known.

Some verbs act like modals in meaning but behave more like normal verbs in grammar, especially have to, be able to, ought to, and need to. They can often express similar ideas to must, can, or should, but they use to and can change with tense in more regular ways. Learning semi-modals helps you express modal meanings in tenses where core modals are limited.

Word/Phrase	Definition	Example
have to	external obligation or requirement	I have to work today.
be able to	ability with flexible tense	She was able to finish.
ought to	advice or expectation, formal	You ought to apologize.
need to	necessity or requirement	We need to leave now.