Read the sentences given below.
- He used to drink, but I never did.
- He can smoke if he wants, but you mustn’t.
- He can swim well, but I can’t.
- She needn’t pay, but you must.
The negative of used to is never did. Usedn’t to is possible but is extremely rare.
When must shows obligation, its negative is needn’t.
When can shows permission, its negative is mustn’t or sometimes mayn’t (= prohibition).
When can shows ability, its negative is can’t.
- Must I wait? You needn’t. (no obligation to wait)
- Can I go? You mustn’t. (prohibition)