ちゃいけない / じゃいけない

ちゃいけない is the spoken form of てはいけない, "must not": you say it to forbid something or warn someone off. Take the て-form, turn て into ちゃ or で into じゃ, and add いけない.

The じゃ verbs are the ones whose て-form ends in んで or いで: のむ飲む, あそぶ遊ぶ, しぬ死ぬ, およぐ泳ぐ.

• × 飲んちゃいけない → 飲んじゃいけない
• × 飲みちゃいけない → 飲んじゃいけない

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ちゃいけない belongs to conversation with family, friends and children. With いけません it stays casual but sounds gentler, the way a teacher talks to a class. In writing, and to someone above you, use the full てはいけない or てはいけません.

ちゃいけない or ちゃう

ちゃう is the spoken form of てしまう and has nothing to do with forbidding. Both start with ちゃ, so listen for what comes after it.

ちゃだめ

だめ can stand in for いけない, giving ちゃだめ and じゃだめ. It is blunter and more casual still. To a small child, ちゃだめ alone is a complete sentence.