〜てはいけない te wa ikenai
〜てはいけない（te wa ikenai）means “must not; may not (prohibition)”. Attaches to the te-form of a verb to express prohibition: 'you must not do X. Formation: V-て + はいけない（口语 ＝ 〜ちゃいけない）
must not; may not (prohibition)
N5Formation
V-て + はいけない（口语 ＝ 〜ちゃいけない）
Explanation
Attaches to the te-form of a verb to express prohibition: 'you must not do X.' It is stronger and more authoritative than just saying not to do something, often used in rules, signs, and from a superior to a subordinate. The polite form is 〜てはいけません.
Examples
Note
In casual speech 〜てはいけない often contracts to 〜ちゃいけない (or 〜じゃいけない after voiced -de). The opposite, granting permission, is 〜てもいい.
FAQ
〜てはいけない — what does it mean and how is it used?
must not; may not (prohibition). Attaches to the te-form of a verb to express prohibition: 'you must not do X.' It is stronger and more authoritative than just saying not to do something, often used in rules, signs, and from a superior to a subordinate. The polite form is 〜てはいけません.
〜てはいけない — how do you form it?
V-て + はいけない（口语 ＝ 〜ちゃいけない）
〜てはいけない — any usage notes?
In casual speech 〜てはいけない often contracts to 〜ちゃいけない (or 〜じゃいけない after voiced -de). The opposite, granting permission, is 〜てもいい.