Prohibition: 〜てはいけない / 〜てはいけません

While ~te mo ii means 'It's okay to do X', the opposite is ~te wa ikenai / ~te wa ikemasen, which means 'It's NOT okay / You must NOT do X'.

This is the standard form used for rules (No photography!), parental warnings (Don't do that!), and official prohibitions.

The casual form is ~te wa dame (da/desu), which is softer and used in everyday speech. The more formal/literary form is ~te wa naranai.

~てはいけません is not the only way to prohibit something in Japanese, and it is not the softest one. In daily conversation between friends, 〜ちゃだめ (casual, from 〜てはだめ) is far more common and sounds like a gentle 'you shouldn't' rather than an official rule. Parents say it to children constantly.

~てはいけません sits in the middle: firm enough for a teacher addressing a class or a sign in public, but not the level you would see in a legal document or an official regulation, where 〜てはならない takes over. Choosing the wrong register is not grammatically wrong, but ~てはいけません from a child to an adult, or 〜ちゃだめ on an official notice, both sound out of place to a native speaker.