Modal Verbs in English: Permissions, Obligations and Advice

Modal verbs do not take -s and they combine with an infinitive without to (exceptions: have to, ought to).

Can / could

• can — ability or permission now: I can swim. · Can I sit here?
• could — past ability, or a more polite request: I could swim when I was five. · Could you open the door?

Must / have to

• must — strong obligation (often the speaker’s): You must stop at a red light.
• have to — obligation from outside: I have to wear a uniform at work.
• No necessity: don't have to (not mustn't). mustn't = prohibition: You mustn't park here.

Should

Advice: You should sleep more. · You shouldn't eat so much sugar.

Formal permission: may — May I come in? (at A2, can / could is enough).