Modal verbs add meanings like ability, possibility, permission, advice, obligation, and prediction. They usually come before the base verb form and do not take -s in the third person singular. Many modals have a “present vs past” relationship in form, but their meanings often extend beyond time into politeness, distance, and uncertainty. This module explains what each modal communicates and how to choose between similar ones.

Modals are followed by the base verb: can go, should try, might happen. For negatives, place not after the modal: cannot, should not, might not, will not. For questions, invert the modal and the subject: Can you help, Should we leave, Will they arrive. Modals typically do not use do-support in questions and negatives.

Rule	Example
Modal + base verb	She can swim.
Modal negative uses not	He should not drive.
Modal questions invert modal and subject	Could you open the window?
No -s in third person singular	She can speak French.

Use can for present ability and general ability. Use could for ability in the past, and also for more tentative or polite ability in the present. For a specific past success, English often prefers was able to over could, especially in affirmative statements. In questions and negatives, could is common for past ability: Could you swim then, I couldn’t lift it.

Rule	Example
Can for present or general ability	I can read fast.
Could for past ability	When I was five, I could ride a bike.
Could is more tentative or polite	Could you help me for a minute?
For a one-time past success, use was able to	We were able to fix it yesterday.

Use can for informal permission and may for more formal or careful permission. Could can request permission politely, often sounding softer than can. Might is rare for permission and usually sounds overly formal or old-fashioned in everyday English. In giving permission, may is common in formal contexts: You may begin.

Rule	Example
Can for informal permission	You can leave early today.
May for formal permission	You may enter the room now.
Could for polite permission requests	Could I use your phone?
Might for permission is uncommon	Might I ask a question?

Use may and might for possibility, with might usually expressing a weaker or more uncertain possibility. Use could for possible outcomes or general possibilities, often meaning “it is possible” rather than “it happened.” Use can for general possibility in the sense of what is sometimes true, not for a specific uncertain event. Adding well often strengthens may or might: It may well rain.

Rule	Example
May for possibility	It may rain later.
Might for weaker possibility	We might miss the bus.
Could for possible outcomes	That could be a problem.
Can for general possibility	Driving at night can be dangerous.

Use should for advice, recommendations, and expectations based on normal logic. Ought to is similar but slightly more formal, though should is far more common. Should can also express that something is likely or expected to be true: They should be home by now. In polite questions, should can ask for guidance: What should I do.

Rule	Example
Should for advice	You should see a doctor.
Should for expected outcome	The package should arrive tomorrow.
Should in questions asks for guidance	Should I call them now?

Use must for strong obligation, strong necessity, or the speaker’s firm rule. Use have to for obligation from external rules or circumstances; in many contexts it is more common than must. Must not expresses prohibition, meaning something is not allowed. Do not confuse must not with do not have to, which means there is no necessity.

Rule	Example
Must for strong obligation	You must wear a helmet.
Must not for prohibition	You must not park here.
Have to for external obligation	I have to work tonight.
Do not have to means no necessity	You do not have to come early.

Must can express a strong logical conclusion about the present: He must be tired. For negative conclusions, English usually uses can’t rather than mustn’t: He can’t be serious. May and might express uncertain conclusions: She may be at work. In the past, use must have, may have, might have to talk about conclusions about earlier events.

Rule	Example
Must for strong logical conclusion	They must be stuck in traffic.
Can’t for strong negative conclusion	He can’t be the manager.
May or might for uncertain conclusion	She might be asleep.
Perfect form for past conclusions	He must have forgotten.

Will is used for predictions, decisions made at the moment of speaking, offers, and promises. Would is the past form in reported speech and is also used for polite requests and hypothetical situations. Shall is mainly used with I and we in formal offers and suggestions, especially in British English, but will is more common overall. For plans and arrangements, English often prefers be going to or the present continuous, but will still works for many predictions and decisions.

Rule	Example
Will for prediction	I think it will snow.
Will for spontaneous decision	I will answer it.
Would as past of will in reporting	She said she would call.
Shall for formal suggestion or offer	Shall we start?

Choosing a modal often changes the level of politeness and directness. Could and would generally sound more polite and less demanding than can and will. May is often more formal than can when asking permission. In requests, would you is a common polite pattern, while could you often emphasizes ability and politeness together.

Rule	Example
Can is direct and informal	Can you send it today?
Could is softer and more polite	Could you send it today?
Will is direct for requests	Will you close the door?
Would is more polite for requests	Would you close the door?
May is formal for permission	May I sit here?

Shall is limited in modern English and is most common in questions with I or we to make suggestions or ask what to do next. In very formal or legal writing, shall can express obligation, but in everyday speech must or will is usually clearer. In American English, shall is relatively rare outside set phrases, while in British English it appears more often in polite suggestions. When in doubt, use will for future and should or must for obligation.

Region	Word	Regional Definition
British English	shall	Common in suggestions with I or we: Shall we begin?
American English	shall	Uncommon in conversation, often replaced by will: Should we begin?
Legal or formal writing	shall	Sometimes used to indicate obligation or requirement