mustn't と don't have to(禁止と不要) Mustn't vs. Don't Have To: Forbidden, or Just Optional?
In the positive they're close cousins; in the negative they split completely. You mustn't go: going is forbidden. You don't have to go: going is optional. Mix them up and real misunderstandings follow.
否定で意味が割れる
Negation Splits Them Apart
Must not makes not-doing the duty: that's prohibition. The not targets the action. Don't have to cancels the duty itself: that's freedom. The not targets the obligation. One little word, two different landing spots, and the meanings fly apart.
You mustn't tell anyone.
You don't have to come if you're busy.
mustn't
してはいけない。 Forbidden.
You mustn't be late for the ceremony.
don't have to
しなくてよい。 Optional.
You don't have to dress up.
You must not pay. It's free.
You don't have to pay. It's free.
You meant "no payment needed," but must not bans paying altogether — a strange order for a free service. For "no need," it's always don't have to (or don't need to).
- You don't have to worry.
心配しなくていいよ。 心配という荷物を下ろさせる、定番の慰め。 The classic comfort line — it takes the load off. - must not be removed
持ち出し禁止(掲示) 掲示の禁止は must not の縄張り。 Signs ban things with must not. - if you must
どうしてもと言うなら 「禁止まではしないが、感心しない」という含みの慣用。 "I won't stop you — but I'm not thrilled." - don't need to
〜する必要はない don't have to とほぼ同義の相棒。どちらも「不要」側。 Don't have to's near-twin — both live on the optional side.
まとめ Summary
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mustn't =禁止、don't have to =不要。正反対。 Mustn't forbids; don't have to frees. Opposites. -
not の掛かり先が違う(行為を消すか、義務を消すか)。 The not hits the action in one, the obligation in the other. -
「無料・不要」の場面で mustn't を使わない。 Never mustn't when you mean "no need." -
don't have to は相手の荷物を下ろす優しい言い方にもなる。 Don't have to doubles as a gentle kindness.