てはいけないから

The structure てはいけないから combines ～てはいけない (must not / it would be bad if) with から (because/since) to express a reason based on preventing a negative outcome. It is used when explaining why you do (or should do) something, by stating the undesirable situation that must be avoided.

The literal meaning is "because [something] must not happen" or "since it would be bad if [something happened]." In natural English, it often translates to "so that ~ doesn't happen", "to make sure ~ doesn't happen", or "because ~ shouldn't happen."

The first clause states the situation that must be avoided, and the second clause states the action taken (or advice given) to prevent it.

Giving advice or warnings

This pattern is used to advise or warn someone to take action in order to prevent a bad result.

Casual spoken form: ちゃいけないから

In casual speech, てはいけない is often contracted to ちゃいけない (or ちゃダメ), so the pattern becomes ～ちゃいけないから.

Related patterns

• ～てはいけない: Expresses prohibition ("must not do").
• ～ないように: Another way to say "so that ~ doesn't happen," but without the strong sense of obligation that てはいけないから carries.
• ～といけないから: A closely related pattern meaning "because it would be a problem if…" which is used interchangeably.