Permission and prohibition
These patterns distinguish what is allowed from what is prohibited. A question asking permission differs from a request for help in who is expected to act.
Ask permission with てもいい
ここに座ってもいいですか。 means 'May I sit here?' Add もいいですか to the te-form of 座る. Normally, the speaker is asking whether they themselves may sit down.
窓を開けてもいいですか。 likewise asks whether the speaker may open the window. Distinguish it from 窓を開けてください, which asks the other person to open it. Identify the actor first.
Prohibit with てはいけない
ここで泳いではいけません。 means 'You must not swim here.' It uses 泳いで, the te-form of 泳ぐ. The name 'te-form' does not mean that every verb literally ends in て rather than で.
〜てはいけません clearly communicates a rule or prohibition. It may sound strong for a personal favor, so distinguish an actual rule from a request you are making on your own behalf.
Prohibited versus unnecessary
行ってはいけません prohibits going. 行かなくてもいいです means that going is not necessary; it does not prohibit it. Keep the distinction explicit instead of reducing both to a vague negative.
When listening to an answer, attend to the explanation after はい or いいえ. If a vague response such as 'It's okay' leaves permission unclear, repeat the specific action in a question such as 座ってもいいですか.
Check: Does 行かなくてもいいです。 mean 'You must not go'?
Show the answer and why
No. It means 'You do not have to go.' To prohibit going, use an expression such as 行ってはいけません.