Express the absence of an obligation.
The rule
Replace ない of the negative form with なくてもいい: "it is not necessary to". This structure does not mean "it is prohibited from".
Examples
あしたこなくてもいいです。
You don't have to come tomorrow.
ぜんぶたべなくてもいいです。
You don't have to eat everything.
Remember
- ～なくてもいい: not necessary; ～てはいけない: prohibited.
- Express the absence of an obligation.
Common mistake
Assimilate a non-obligation to a prohibition.