なくてもいい

[No obligation: don't have to ~]

Connection: Verb ない-form (drop い) + くてもいい

How to form:

• Group 2: 食べる → 食べない → 食べなくてもいい (don't have to eat)
• Group 1: 行く → 行かない → 行かなくてもいい (don't have to go); 書く → 書かない → 書かなくてもいい (don't have to write)
• Irregular: する → しない → しなくてもいい (don't have to do); くる → こない → こなくてもいい (don't have to come)

Note: This means "it's OK even if you don't" — absence of obligation, NOT prohibition.

• 行かなくてもいい (You don't have to go — it's optional)
• 行ってはいけない (You must not go — it's forbidden)

Polite form: なくてもいいです / なくてもかまいません

Examples:

• 明日は来なくてもいいですよ (You don't have to come tomorrow)
• 全部食べなくてもいいです (You don't have to eat everything)
• 急がなくてもいいです (You don't have to hurry)