Permission & Prohibition in Japanese: 〜てもいい, 〜てはいけない
Asking and granting permission, expressing what is forbidden, and describing obligations are core communicative functions. Japanese uses specific te-form patterns for all three.
〜てもいい — permission (may, can)
〜てもいい means "it's OK to do X" or "you may do X". To ask permission, add ですか. The negative 〜てはいけない means the opposite.
〜てはいけない / 〜てはだめ — prohibition
〜てはいけない means "must not" or "you may not". 〜てはだめ is more casual with the same meaning. In speech, てはいけない often contracts to ちゃいけない.
〜なければならない — obligation (must)
〜なければならない (often contracted to 〜なきゃ in speech) means "must do" or "have to do". It expresses a strong obligation.
〜なくてもいい — no need to
〜なくてもいい means "you don't have to" or "it's OK not to". It negates obligation without creating prohibition.
Common mistakes
行ってもいいですか to superiors in formal contexts
RightUse 〜させていただいてもよろしいでしょうか for polite requests with superiors
〜てもいいですか is neutral-polite. In business Japanese, asking your boss requires humble forms: 「先に失礼させていただいてもよろしいでしょうか？」
行かなければいけない vs 行かなければならない — thinking they're different
RightBoth mean "must go"; いけない is slightly more conversational, ならない more formal
These are interchangeable in most contexts. ならない tends to appear in writing; いけない in speech. Both are acceptable at N4–N3 level.
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How do I say "you are not allowed to" vs "you don't have to"?
"You are not allowed to" = 〜てはいけない (prohibition — must NOT). "You don't have to" = 〜なくてもいい (no obligation — OK not to). These are opposites, so mixing them up is a serious error.
What is the casual contraction of 〜なければならない?
The most common casual contractions: なければ → なきゃ, ならない → (dropped or ね). So 行かなければならない → 行かなきゃ (must go). In very casual speech: 行かなくちゃ is also used.