ないでください

1. 1Politely asks the listener not to perform an action.Ex. 1
2. 2States a rule or prohibition in a courteous way, as in notices or classroom instructions.Ex. 2

This is a polite, everyday way to ask someone not to do something, and it is common in speech, signs, and instructions. It sounds softer than a direct prohibition such as てはいけない, because it is framed as a request rather than a statement of what is not allowed. Friends and family often drop ください and simply say ~ないで.

• てはいけないN5「てはいけない」 attaches to the て-form and directly declares a prohibition with a stronger tone, often used for rules or toward subordinates. When prohibiting someone else, the two are interchangeable, but 「てはいけない」 can also be used to warn yourself, and it can attach to adjectives and nouns to mean '...is not acceptable.'
• なN5「な」 also expresses prohibition, but in a rough, forceful tone; it attaches to the verb's dictionary form (行くな) and appears mostly in masculine speech, from superiors to subordinates, in urgent shouts to stop someone, and on warning signs, so it is inappropriate toward elders or superiors. In polite situations the two are not interchangeable.
• てはならないN2「てはならない」 attaches to the て-form with a solemn, written tone, seen mostly in laws and moral instruction, and it declares a norm rather than making a request. It can also warn that something must never happen — a use 「ないでください」 lacks — so for everyday requests not to do something, use 「ないでください」.
• べからずN1「べからず」 attaches to the dictionary form and is a classical-style written prohibition seen mostly on notices and in regulations, while 「ないでください」 attaches to the ない-form and is a polite, request-style prohibition. Their meanings are similar on notices, but their tones are very different, and 「べからず」 should not be used in conversation.
• なくてもいいN4「なくてもいい」 means 'it's fine not to do it' or 'it's fine even without something or not being in a certain state,' while 「ないでください」 asks the other person not to do something. The two cannot be swapped, and 「なくてもいい」 can also attach to adjectives and nouns.

ないでください is a polite request not to do something, while てはいけない states that an action is not allowed, and な is a blunt, rough command. てはならない and べからず are formal or written prohibitions, the latter classical and typical of signs. なくてもいい means 'you don't have to' and expresses a lack of obligation, not a request to refrain.