てはいけない details

write + (and…) + must not do

kaite wa ikenai

must not do; prohibited from doing

meaning: must not write

それ、まだ誰だれにも言いっちゃいけないよ。

sore, mada dare ni mo itcha ikenai yo.

You can't tell anyone about that yet. casual

この薬くすりを飲のんだ後あとは、車くるまを運転しうんてんしてはいけない。

kono kusuri o nonda ato wa, kuruma o untenshite wa ikenai.

You must not drive after taking this medicine. neutral

Examples

must not do; prohibited from doing

それ、まだ誰だれにも言いっちゃいけないよ。

sore, mada dare ni mo itcha ikenai yo.

You can't tell anyone about that yet. casual

この薬くすりを飲のんだ後あとは、車くるまを運転うんてんしてはいけない。

kono kusuri o nonda ato wa, kuruma o untenshitehaikenai.

You must not drive after taking this medicine. neutral

会場内かいじょうないでは写真しゃしんを撮とってはならない。

kaijounai de wa shashin o totte wa naranai.

Photography is prohibited inside the venue. formal

ないで
Both stop the listener — てはいけない states the rule bluntly; ないで(ください) frames the same stop as a request, which is what strangers actually hear.
ないでください
Both stop the listener — てはいけない states the ban as a rule; ないでください delivers it as a polite request, which is why public signage prefers ないでください as its outward face.
ないほうがいい
ないほうがいい advises against and leaves the choice with the listener; てはいけない removes the choice by rule — a doctor softens 飲んではいけません into 飲まないほうがいいです only when it truly is just advice.

てもいいprohibition-vs-permission
Same て-frame one kana apart — も grants ("may do"), は forbids ("must not do").
ないといけないprohibition
Obligation applied to the negation instead — "you must not."

なくてはいけない
Shared てはいけない tail, opposite forces — てはいけない forbids the act, なくてはいけない mandates it; the なく in front flips prohibition into obligation.

no other patterns in this meaning class yet