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Negative-Form VERB + なくて(はいけない/もいい) as “(X must do VERB/it’s okay if X doesn’t do VERB)”

Negative-Form VERB + なくて(はいけない/もいい) as “(X must do VERB/it’s okay if X doesn’t do VERB)”

This grammar construct is similar to the

て-Form VERB + もいいですか as “is it okay to do VERB?”
て-Form VERB + もいい as “it is okay to do VERB”
て-Form VERB + はいけない as “it is not okay to do VERB”

series of grammar patterns in that the phrases “もいい” and “はいけない” are similarly implemented. Additionally, in grammatical construction, this construct is essentially the て-Form of the ない-Form of a VERB followed by the same phrasing. Literally, Negative-Form VERB + なくてはいけない translates to “it is not okay to not do VERB” and Negative-Form VERB + なくてもいい translates to “it’s fine to not also do VERB.”

Example(s):

明日パーティーに行きますから、今日ちゃんと勉強しなくてはいけません
Ashita party ni ikimasu kara, kyou chanto benkyou shinakute wa ikemasen.
(I’m going to a party tomorrow so I have to study well today.)

妹は家にいますから帰らなくてはいけません
Imouto wa uchi ni imasu kara kaeranakute wa ikemasen.
(My sister is at home right now so it’s not okay if I don’t return.)

明日仕事がありませんから今夜早く寝なくてもいいです。
Ashita shigoto ga arimasen kara konya hayaku nenakute mo ii desu.
(I don’t have work tomorrow so it’s fine if I don’t go to sleep early.)

浴びなくてもいいですか?
Abinakute mo ii desuka?
(Is it okay if I don’t shower?)

いいえ、浴びなくてはいけませんよ
Iie, abinakute wa ikemasenyo.
(No, you have to shower!)