Modal verbs express meanings like ability, possibility, permission, necessity, and intention. Modals do not take -s in the third person singular. Modals are followed by the base form of the main verb. Questions and negatives with modals use do not, because modals already act as auxiliaries.

Rule
Modal verbs are followed by the base form of the verb.
Modal verbs do not take -s in the third person singular.
With modals, questions and negatives use the modal directly, not do or does.

Can expresses present ability or general capability. Could expresses past ability or a more tentative ability. Both are followed by the base form of the main verb.

Word/Phrase	Definition
can	Can expresses present ability or general capability.
could	Could expresses past ability or more tentative ability.
be able to	Be able to expresses ability and can be used where tense or aspect is needed.

May and might express possibility, with might often sounding less certain. Could can also express possibility, often in hypothetical or conditional contexts. The choice between may and might is about degree of likelihood, not grammar.

Word/Phrase	Definition
may	May expresses possibility with a neutral or slightly higher likelihood.
might	Might expresses possibility with a lower or more tentative likelihood.
could	Could expresses possibility, often in hypothetical or conditional contexts.

Can is used for informal permission in many contexts. May is used for formal or polite permission. Could is used for polite requests, which functions like indirect permission.

Word/Phrase	Definition
can	Can expresses informal permission.
may	May expresses formal or polite permission.
could	Could forms polite requests, functioning as indirect permission.

Must expresses strong necessity or obligation from the speaker. Have to expresses necessity, often as an external requirement. Need to expresses necessity and is treated like a regular verb, not a modal, in most uses.

Word/Phrase	Definition
must	Must expresses strong necessity or obligation from the speaker.
have to	Have to expresses necessity as an external requirement.
need to	Need to expresses necessity and behaves like a regular verb.

Must not (mustn’t) expresses strong prohibition. Cannot (can’t) expresses that something is not allowed or not possible. May not expresses formal prohibition, especially in official or written contexts.

Word/Phrase	Definition
must not	Must not expresses strong prohibition.
cannot	Cannot expresses that something is not allowed or not possible.
may not	May not expresses formal prohibition.

Should expresses advice or recommendation. Ought to also expresses advice and is slightly more formal. Both are followed by the base form, and ought to includes to as part of the structure.

Word/Phrase	Definition
should	Should expresses advice or recommendation.
ought to	Ought to expresses advice and is slightly more formal.
had better	Had better gives strong advice, often with a sense of warning.

Will expresses prediction or certainty about the future. Going to expresses prediction based on present evidence or intention. Would can express future in the past or hypothetical prediction.

Word/Phrase	Definition
will	Will expresses prediction or certainty about the future.
going to	Going to expresses prediction based on present evidence or intention.
would	Would expresses future in the past or hypothetical prediction.

Could and would are used for polite requests and offers. May is used for very formal or polite permission. Will is used for offers and requests, and sounds more direct than would or could.

Word/Phrase	Definition
could	Could is used for polite requests.
would	Would is used for polite offers and requests.
may	May is used for very formal or polite permission.

With modals, questions are formed by placing the modal before the subject. The main verb remains in the base form. Negatives are formed by adding not after the modal.

Rule
Place the modal before the subject to form a question.
Use the base form of the main verb after the modal in questions.
Add not after the modal to form a negative.

Could is used for general past ability. For past possibility or speculation, English uses may have, might have, and could have. For past deduction, English uses must have and can’t have.

Word/Phrase	Definition
could	Could expresses general past ability.
might have	Might have expresses past possibility or speculation.
must have	Must have expresses past deduction with high certainty.

Modal verbs in English express meanings like ability, possibility, permission, necessity, advice, and prediction. Modals are followed by the base form and do not take -s in the third person. Different modals and phrases are chosen based on meaning and formality. Consistent structure and meaning allow clear and correct use of modals.