～てはいけない

てはいけない means "must not": the action is not allowed. It goes on the て-form of the verb, followed by は. In polite speech it ends in いけません.

A verb whose て-form ends in で gives ではいけない.

てはいけない or ないといけない

The two look alike and mean opposite things. てはいけない forbids an action; ないといけない and なければいけない make it compulsory.

Mixing them up reverses the meaning: くすり薬をのんでは飲んではいけない is "you must not take the medicine", and くすり薬をのまなくては飲まなくてはいけない is "you have to take it".

Casual forms

In conversation ては shrinks to ちゃ and では to じゃ. だめ often replaces いけない, and with family and children it is the everyday way to say "don't".

Usage notes

These go beyond spoken prohibitions.

てはならない

てはならない means the same and belongs to rules, laws and written instructions. In conversation it sounds stiff.

禁止 on signs

Signs and notices use a noun with きんし禁止, "prohibited", instead of a sentence.