てはいけない

「〜てはいけない」 expresses prohibition, 'you must not …': 「ここに車を止めてはいけません」 (You must not park here). It is used for laws, rules, school regulations, or elders teaching the young, in a firm tone. The polite form is 「〜てはいけません」, and the spoken contractions are 「〜ちゃいけない」 and 「〜じゃいけない」.

•It carries a tone of authority from above, so do not say it to an elder, a boss or a client. To discourage someone in business, use 「〜はご遠慮ください」 or 「お控えください」.

•It is the standard negative answer to 「〜てもいいですか」: asked 「タバコを吸ってもいいですか」, answer 「いいえ、吸ってはいけません」.

•In listening you often hear the contractions 「〜ちゃだめ」 and 「〜ちゃいけない」, which come from 「ては」: 「忘れちゃいけない」 (you mustn't forget).