Explanation
Use "must" when you are almost sure about something from clues. It is a strong guess, not something you saw directly.
For example, "The window is open, so it must be cold in here." In this sentence, "The window is open" is the clue, and "must be cold" is the conclusion. Japanese often uses "〜にちがいない" or "きっと" here, but English puts the guess inside the verb phrase with "must." If you mean the past, use "must have + past participle," as in "They must have left already."
手がかりから「ほぼ確実だ」と言いたいときに "must" を使います。直接見た事実ではなく、見えている状況からの強い推測です。
たとえば "The window is open, so it must be cold in here." では、"The window is open" が手がかりで、"must be cold" がそこから出した結論です。日本語では「〜にちがいない」「きっと」に近いですが、英語では "must" を使って文の中で推量を表します。過去のことを言うなら "must have + 過去分詞" を使い、たとえば "They must have left already." のように言います。
Examples
- The window is open, so it must be cold in here. 窓が開いているので、ここは寒いにちがいありません。
- She is smiling, so she must be happy. 彼女は笑っているので、きっと幸せなのでしょう。
- Who is that man? He must be Ken's father. あの人はだれですか。ケンのお父さんにちがいありません。
- Why is the floor wet? The dog must have come inside. どうして床がぬれているの？ 犬が中に入ってきたにちがいありません。
- The car is gone. They must have left already. 車がありません。もう出かけたにちがいありません。
- The lights are off, so they must not be home. 電気が消えているので、まだ家にいないのでしょう。
Common Mistakes
Japanese learners often confuse deduction with obligation, or they forget "be" and "have."
Wrong: "He must tired."
Right: "He must be tired."
Wrong: "They must left already."
Right: "They must have left already."
日本人学習者は、推量の "must" と義務の "must" を混同しやすく、さらに "be" や "have" を落としやすいです。
誤り: "He must tired." → 正しくは "He must be tired."
誤り: "They must left already." → 正しくは "They must have left already."