てはいけない

1. 1Prohibits an action, stating that someone must not or may not do it.Ex. 1
2. 2States that a condition or state is unacceptable or not allowed.

Neutral in register; the polite てはいけません is common in rules, notices, classrooms, and when parents or teachers say what not to do. Because it states a rule with authority, it is usually directed downward or used for general rules, and casual speech often shortens it to ちゃいけない or じゃいけない. It is firmer than the request ないでください and less formal than the written てはならない.

• てはならないN2「てはならない」 expresses only prohibition and admonition, and is stiffer and more written-style, appearing often in laws, admonitions, and editorials; it can also say morally that something 'should never happen,' and is rarely used in daily conversation. Nouns and な-adjectives attach as 「であってはならない」.
• ないでくださいN5「ないでください」 attaches to the ない-form and states a prohibition as a request, with a softer tone; it is used only to ask someone else not to do something and cannot be used to admonish yourself, and it attaches only to verbs, so it cannot mean 'must not be (in some state).' When prohibiting someone else, the two are interchangeable, differing only in tone.
• なN5The prohibitive 「な」 attaches only to the dictionary form, not the ます-form, and sounds rough, appearing mostly in spoken rebukes and signs; it is interchangeable with 「てはいけない」 but harsher, and should not be used with elders or superiors. A sentence-final 「な」 also has separate exclamatory and confirmatory uses.
• べからずN1「べからず」 is a classical written form that attaches only to the verb dictionary form, seen mostly in notices and regulations and almost never in speech; the related 「べからざる」 can modify nouns. It overlaps with 「てはいけない」 only in the sense of prohibition and cannot express that a state is inappropriate.
• べきではないN3「べきではない」 emphasizes that something should not be done by reason or morality, used in argument, advice, and criticism, with a more preachy tone; its past form 「べきではなかった」 can also express regret. For simple rule-based prohibition, 「てはいけない」 is more natural.
• てもいいN5「てもいい」 means the opposite, expressing permission or 'it's fine to...,' and attaches to verbs, adjectives, and nouns. When refusing a request for permission, saying 「てはいけません」 to a superior sounds stiff, so a soft reply like 「ちょっと……」 can be used instead.

てはならない is a more formal, written counterpart used for rules and moral duties, and べからず is an archaic form found on signs. ないでください is a polite request not to do something, な is a blunt direct command, べきではない advises that something should not be done, and てもいい grants permission, the opposite meaning.