Grammar Info
N4 Lesson 6: 14/16

なくてもいい
don't have to

も can be dropped in casual speech

Structure
Casual
Formal
Verb[なくて] + ( + いい
Verb[なくて] + ( + いい + です
Details
Register Standard
使用域 一般
Information
なくてもいい is simply the opposite of てもいい, and highlights that it is ‘ok even if (A) is not done’. This usually comes across as being quite polite, and sounds like ‘don’t worry about doing (A)’ in English.
なくてもいい is a combination of the auxiliary verb ない in its conjunction form, with てもいい.
  • ミカ悪(わる)くないから謝(あやま)らなくてもいい
    Mika, you are not in the wrong, so you don't have to apologize.
  • その手紙(てがみ)もう届(とど)けなくてもいい
    It's okay if you don't deliver that letter anymore.
In order to make this phrase sound even more casual, the adverbial particle may be omitted. This does not change the nuance at all, but will sound a bit more friendly (when used with people that you know well).
  • 今日(きょう)会社(かいしゃ)に来(こ)なくていい
    It is fine if you don't come to the office today.
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忙(いそが)しいなら行(い)かなくてもいいです
If you are busy, you don't need to go.
[it is okay even if you don't]
掃除(そうじ)なくてもいいです
I don't have to clean?
[Is it okay even if I don't?]
Play  
その青(あお)いセーター着(き)なくてもいいです
You don't have to wear that blue sweater.
[it is okay even if you don't]
Play  
Books
Genki II 1st Edition
Page 99
みんなの日本語 I
Page 110 [CH 17]
Genki II 2nd Edition
Page 125