Obligation, prohibition and permission

Use must, have to, must not, do not have to and can to describe rules, necessity and permission.

When do we use it?

Distinguish something necessary, forbidden, optional or allowed.

Form

Must/have to = necessary; mustn’t = forbidden; don’t have to = not necessary; can = permitted.

• You must wear a helmet.
• You mustn't smoke.
• You don't have to come.
• You can leave early.

Key contrast

Read the whole context and identify the time, meaning, noun type or sentence structure before choosing.

Common mistakes

• You don't have to smoke here. → You mustn't smoke here.
• You mustn't come if you're busy. → You don't have to come if you're busy.

Remember

Use must, have to, must not, do not have to and can to describe rules, necessity and permission.

Quick check

• Find the grammar clue.
• Check the subject and verb form.
• Confirm that the answer fits both structure and meaning.