なくてもいい details

write + don't have to do

kaka nakute mo ii

don't have to do; it's okay not to do

meaning: don't have to write

明日あしたはこなくてもいいよ。

ashita wa ko nakute mo ii yo.

You don't have to come tomorrow. casual

靴くつは脱ぬがなくてもいいですか。

kutsu wa nuganakute mo ii desu ka.

Is it okay if I keep my shoes on? neutral

Examples

don't have to do; it's fine not to — waiving an obligation

明日あしたはこなくてもいいよ。

ashita wa konakutemoii yo.

You don't have to come tomorrow. casual

靴くつは脱ぬがなくてもいいですか。

kutsu wa nuganakute mo ii desu ka.

Is it okay if I keep my shoes on? neutral

詳細しょうさいは今いま決きめなくてもかまいません。

shousai wa ima kimenakute mo kamaimasen.

You need not decide the details right now. formal

なくてもだいじょうぶ — usually written なくても大丈夫[だいじょうぶ] — reassures rather than rules ("it'll be fine even if you don't")
なくてもかまわない — usually written なくても構[かま]わない — "(I) don't mind if you don't"; cooler and more detached than なくてもいい

The natural reply to 行かなければなりませんか is いいえ、行かなくてもいいです — never a direct negation of なりません.

てもいい
Matched pair sharing the もいい shell — てもいい permits DOING, なくてもいい permits NOT doing; each grants latitude over the same act from the opposite side.

ないといけないnegated-obligation
Negates the obligation itself — "you don't have to."
なければならないnegated-obligation
なければならない obliges the act ("must do"); なくてもいい waives it ("need not do") — the natural answer to 行かなければなりませんか is 行かなくてもいいです.

なくて
なくて alone links a reason or contrast ("not doing, so…"); adding もいい turns it into an exemption — the shared なくて opening invites misreading mid-sentence.

Pattern	Attaches to
`koto-wa-nai` (planned)	—