てはいけない

てはいけない states the rule bluntly. ないで(ください) turns the same stop into a request, which strangers actually hear.

てはいけない states a standing rule for anyone (館内かんないでは走はしってはいけない). な suits a shout or a sign, never a rule.

てはいけない states the ban as a flat rule. ないでください delivers it as a polite request, which is why signs favour it.

ないほうがいい advises against something, and leaves the choice with the listener. てはいけない removes the choice with a flat rule.

てはいけない is a rule from someone else, like a parent. わけにはいかない is your own reason, from the situation.

も grants doing it ("may do"). は forbids it instead ("must not do").

てはいけない bans the act ("you must not"). ないといけない instead requires it ("you must").

てはいけない forbids the act. なくてはいけない mandates it instead, with なく flipping prohibition into obligation.

てはいけない is a fixed prohibition, always followed by a bad judgment. ては〜ては is a bare repeat joint carrying no judgment at all.

てはいけない is a fixed prohibition, forbidding someone something (ここで写真しゃしんを撮とってはいけない). Bare ては just states a condition, with any subject, forbidding nobody anything (雨あめが降ふっては中止ちゅうしだ).