Modal Verbs: Meaning and Use

Modal verbs are helping verbs that add a meaning such as ability, permission, possibility, obligation or advice to the main verb.

Grammar rules for all modals

• A modal is followed by the base form of the verb: She can swim, not "can swims" or "can to swim".
• Modals do not take -s in the third person: He must go.
• Questions and negatives are made without do: Can you help? You should not worry.
• Ought to is the exception that keeps "to".

Meanings

• Can — ability, informal permission: She can speak French. Can I sit here?
• Could — past ability, polite request, possibility: He could read at four. Could you open the door?
• May — formal permission, possibility: May I come in? It may rain today.
• Might — weaker possibility: He might come, but I doubt it.
• Must — strong obligation, or a logical certainty: You must wear a seat belt. She must be tired after the journey.
• Mustn't — prohibition: You mustn't smoke here.
• Needn't / don't have to — no obligation: You needn't bring food; it is provided.
• Should / ought to — advice, duty: You should revise daily. We ought to respect elders.
• Will / would — future, willingness, polite requests: Would you like some tea?
• Shall — offers and suggestions with I/we: Shall we begin?

Modals about the past (modal + have + V3)

Mustn't vs needn't