JLPT N5 Must, may, and must not
てもいいです grants permission and なくてもいい removes an obligation, while てはいけない・ちゃいけない forbid an action. なくてはいけない, なくてはならない, ないといけない, and casual なくちゃ all mean "must" at different levels of formality. The double negative confuses many learners: read なくては as "if you don't" plus いけない "it's no good".
8 N5 grammar points in this group
てもいいです (temo ii desu): it is OK to; may I
Break the pattern into pieces and the meaning is easy to see:
すみません、ここに座ってもいいですか。 — Excuse me, may I sit here?
なくてもいい (naku temo ii): don't have to
The pattern has three parts: the ない-form of a verb, ても ("even if"), and いい ("good, fine").
明日は学校に行かなくてもいい — You don't have to go to school tomorrow.
てはいけない (te wa ikenai): must not; may not
Break the pattern into pieces and the meaning makes sense:
ここで写真を撮ってはいけません。 — You can't take photos here.
ちゃいけない・じゃいけない (cha ikenai ja ikenai): must not do
〜ちゃいけない tells someone that an action is not allowed. The speaker is either stating a rule (“you can't swim in this river”) or warning someone in the moment (“don't eat that yet!”).
ここで写真を撮っちゃいけない。 — You can't take photos here.
なくてはいけない (nakute wa ikenai): must do; need to do
Split the pattern into its parts:
毎日、日本語を勉強しなくてはいけません。 — I have to study Japanese every day.
なくてはならない (nakute wa naranai): must do; have to do
なくてはならない expresses necessity or duty.
約束は守らなくてはならない。 — Promises must be kept.
ないといけない (naito ikenai): must do
Break the pattern into pieces:
明日は早く起きないといけない。 — I have to get up early tomorrow.
なくちゃ (nakucha): must do; gotta do
なくちゃ expresses obligation or necessity: something you need to do, often right now.
もう寝なくちゃ。 — I've got to go to bed now.