Saying something is not allowed: てはいけません
Use a clear prohibition when an action is not allowed.
Coco-sensei
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Use te-form + は + いけません to prohibit an action. ここ で 食べては いけません means “You must not eat here” or “Eating here is not allowed.”
Compare the endings: ても いいです gives permission; ては いけません expresses a prohibition. If the te-form ends in で, keep it: 飲んでは いけません.
This is a firm expression, useful for rules and instructions. It can sound too strong for a casual request between friends. It means “must not,” not “do not have to.” Those English expressions describe very different things.
Examples
ここ で 写真 を 撮っては いけません。
You must not take photos here.
お酒 を 飲んでは いけません。
You must not drink alcohol.
図書館 で 話しては いけません。
Talking in the library is not allowed.
ここ に 入っては いけません。
You must not enter here.