てはならない

1. 1Prohibits an action on the basis of rules or regulations.Ex. 1
2. 2States a moral principle or general admonition about what one must never do.Ex. 2

This is a formal, written-style prohibition found in rules, regulations, notices, speeches and statements of moral principle. It sounds weightier and more impersonal than the everyday てはいけない. It usually states a general rule rather than a remark aimed at one particular listener in conversation.

• 1.
  てはいけないN5Neutral in tone and common in speech, 「てはいけない」 can contract to 「ちゃいけない」, can also express that a state is inappropriate, and takes 「ではいけない」 after nouns and な-adjectives. The two are interchangeable for prohibitions, but 「てはいけない」 is less solemn than 「てはならない」.
• 2.
  べからずN1Carrying a classical flavor, 「べからず」 attaches only to the verb's dictionary form and is shorter than 「てはならない」, reading more like a posted notice. The two are interchangeable for rule-based prohibitions, but 「べからず」 cannot attach to adjectives or nouns—use 「べからざる」 and the like to modify nouns.
• 3.
  ものではないN2「ものではない」 attaches to the verb's dictionary or potential form and, in a neutral tone, politely advises on common-sense grounds that one 'shouldn't' do something—only in this sense is it close to the admonishing meaning of 「てはならない」. It also means 'there's no way one can…,' and in that sense it cannot be replaced with 「てはならない」.
• 4.
  べきではないN3「べきではない」 attaches to the verb's dictionary form and, in a neutral tone, expresses the speaker's reasoned judgment that something 'shouldn't' be done—like 「てはならない」 for moral admonition, though for rule-style 'not allowed' 「てはならない」 is the better choice. Its past form 「べきではなかった」 can express regret.
• 5.
  まじきN1A classical leftover, 「まじき」 usually only modifies nouns attributively and cannot end a sentence; it carries a stronger tone of condemnation and commonly appears as 「名詞＋にあるまじき＋名詞」. 「あってはならない事態」 can be intensified to 「あるまじき事態」.
• 6.
  ないでくださいN5「ないでください」 attaches to the ない-form and politely asks the listener not to do something; neutral in tone, it is common in both everyday speech and public notices. 「てはならない」 attaches to the て-form, directly proclaims the rule, is more formal and solemn, and can also warn that something must never happen.

てはいけない is the everyday conversational prohibition, and ないでください is a polite request not to do something, whereas てはならない is a formal, rule-like ban. べからず is a classical form typical of signs, ものではない gives advice based on social norms, べきではない states the speaker's opinion that something is wrong, and まじき (as in あるまじき) is an attributive form meaning 'unbefitting'.