Grammar Info
N5 Lesson 9: 7/13

~てはいけない
Must not, May not

Structure
Casual
Formal
Verb[て]+ は + いけない
Verb[て]+ は + いけません
Details
Part of Speech Verb
Word Type Auxiliary Verb
Register Formal
品詞 動詞
単語の種類 助動詞
使用域 硬い
Information
てはいけない is used in Japanese to express that (A) ‘cannot’, or ‘must not’ be done. It is the combination of a verb in form, the particle, and the negative potential form of 行(い)く (to be able to go). In this way, it literally means ‘(A) cannot/is not able to go’.
  • タクシー扉(とびら)自分(じぶん)開(あ)けてはいけない
    You must not open the taxi door by yourself.
  • 子供(こども)前(まえ)タバコ吸(す)ってはいけません
    You must not smoke cigarettes in front of children.
Although can be dropped from many phrases and expressions, the in this particular grammar point is usually not. This is because いけない would then become the second action in a sequence of events (the usual function of form). However, to make this more casual, is often changed to ちゃ (ては becomes ちゃ), or じゃ (では becomes じゃ), as this is easier to say.
  • 危(あぶ)ない場所(ばしょ)行(い)っちゃいけない
    You must not go to dangerous places.
  • 旗(はた)踏(ふ)んじゃいけない
    You must not step on a flag.
Caution
Apart from いけない; ならない and だめ are also frequently used to get across the same meaning. However, they are used in different situations.
いけない - Fairly neutral. Used in both spoken and written language.
ならない - Fairly formal. Used mostly in written language.
だめ - Very casual. Used almost exclusively in spoken language.
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そこ入(はい)ってはいけない
You must not go in there.
Play  
クジラ食(た)べてはいけない
We must not eat whale.
Play  
このビル入(はい)ってはいけない
You must not go into this building.
Play  
Books
Genki I 2nd Edition
Page 152
みんなの日本語 I
Page 98 [CH 15]
Marugoto Elementary 2 (A2) Rikai
Page 48
Genki I
Page 120
Tae Kim's Japanese Grammar Guide
Page 140
[DBJG] A Dictionary of Basic Japanese Grammar
Page 528