確信の must be / can't be Must Be, Can't Be: The Logic Modals
助動詞 Modal Verbs
You must be tired. は「疲れているに違いない」、That can't be true. は「本当のはずがない」。義務の must とは別の顔=証拠から結論を出す論理の推量です。急所は否定側: 「〜のはずがない」は mustn't ではなく can't。
You must be tired — "you've got to be." That can't be true — "no way it is." This is must's other face: drawing conclusions from evidence. The trap is on the negative side — "that's impossible" takes can't, not mustn't.
証拠→結論の must
From Evidence to Conclusion
目の前の証拠から「そうとしか考えられない」と結論する must be。逆に「そう考えると矛盾する」と締め出す can't be。確信の度合いはどちらも最強クラスで、may/might(かもしれない)のずっと上に位置します。過去のことなら must have done(〜したに違いない)。
Must be concludes from the evidence: no other explanation fits. Can't be rules one out: that explanation breaks the facts. Both sit at the top of the certainty scale, far above may and might. For the past, the pattern extends: must have done — "no doubt that's what happened."
確信の言い方 Certainty, spoken aloud
You've been driving all day — you must be exhausted.
一日中運転してたんだから、くたくたに違いないよ。
証拠(一日中運転)→結論。 Evidence (all that driving) → conclusion.
He can't be at home. The lights are off.
家にいるはずがない。電気が消えてる。
証拠が可能性を締め出す can't。 The evidence slams the door — can't.
That must have been hard.
大変だったでしょうね。
過去への確信は must have done。共感の定番でもある。 Certainty about the past — and a staple of sympathy.
must be
〜に違いない(肯定の確信)。 "No other explanation" — positive certainty.
There must be some mistake.
何かの間違いに違いない。
can't be
〜のはずがない(否定の確信)。 "That one's impossible" — negative certainty.
It can't be five already!
もう5時のはずないでしょ!
よくある間違い Common Mistake
He mustn't be at home.
家にいないに違いない。(のつもりで)
He can't be at home.
家にいるはずがない。
推量の否定は can't が標準です。mustn't だと禁止(いてはいけない)に聞こえます。なおアメリカ英語では推量の must not(He must not be home.)はごく普通に使われますが、地域を選ばない基準としては can't で覚えるのが安全です。
The standard negative for deduction is can't. Mustn't reads as prohibition — "he's not allowed to be home." American English deduces with must not quite freely (He must not be home), but as your region-proof default, can't is the safe bet.
- You must be joking. 冗談でしょう。 「冗談としか考えられない」という推量が、驚きの決まり文句に。 "The only explanation is a joke" — deduction fossilized into disbelief.
- It can't be helped. 仕方がない。 「救いようがあるはずがない」→「どうしようもない」。日本語の「しょうがない」の定訳。 "No fix is possible" — the standard English for Japanese しょうがない.
- must have done 〜したに違いない 過去に向けた確信。You must have been surprised.(驚いたでしょう) Certainty aimed at the past: You must have been surprised.
- There must be a way. きっと方法があるはずだ。 希望を論理の形で言う励ましの定番。 Hope dressed as logic — the classic encouragement.
まとめ Summary
- 証拠からの確信: 肯定は must be、否定は can't be。 Evidence-based certainty: must be up, can't be down.
- mustn't は禁止に聞こえる(推量の否定には使わない)。 Mustn't reads as prohibition — keep it out of deductions.
- 過去への確信は must have done。 Past certainty: must have done.
- You must be ○○! は歓迎のあいさつにもなる。 You must be …! doubles as a welcome.