推斷：must / can't

根據證據進行推測時，如果你確定某事是真的，用 must；如果確定它不是真的，用 can't。介於兩者之間的是 might / may / could（有可能）。它們後面都接動詞原形。

• must / can't + be + 形容詞、名詞、地點：He must be at work。
• must / can't + 動詞原形（常是 know、have、live、like…）：She must know him。
• must / can't + be + -ing 表示正在進行的事：They must be watching the match。

💡只有證據充分、你幾乎可以肯定時才用 must。把握不大的猜測，用 might、may 或 probably。

根據你看到、聽到或知道的情況推斷出結論時，用 must。證據常常就在同一句或下一句裡。

• You've been on your feet all day. You must be exhausted.

  你站了一整天，一定累壞了。

• The lights are off. They must be out.

  燈都關著，他們肯定出去了。

• Minh must know the answer — he's a maths teacher.

  阿明一定知道答案——他是數學老師。

• Listen to that noise. The neighbours must be having a party.

  聽那動靜，鄰居肯定在開派對。

• You must be Lan's brother. You look just like her!

  你一定是阿蘭的哥哥吧，你們長得太像了！

推測時，must 的否定是 can't（或 couldn't），不是 mustn't——mustn't 的意思是“不允許”。美式英語中人們也說 must not（不縮寫）表示“我推斷這不是真的”：He must not be home——但 can't 到哪兒都通用，是最穩妥的選擇。

• That can't be Tuan. He's in Singapore this week.

  那不可能是阿俊，他這週在新加坡。

• Ten dollars for a new laptop? That can't be right.

  十美元買臺新筆記型電腦？不可能吧。

• She can't be hungry — she's just had a big lunch.

  她不可能餓——她剛吃了一頓豐盛的午飯。

• You can't be serious!

  你不是在開玩笑吧！（表示驚訝的固定說法）

💡這裡的 “can't” 不是說能力（I can't swim）；它的意思是“這不可能 / 不是真的”。

同一個詞，兩種意思。表示推測時通常接 be、be + -ing 或狀態動詞（know、have、like、live）；表示規定時通常接動作動詞——但一定要看上下文。另見“義務：must / have to / need to”一課。

推測已經發生的事，用 must have + V3 和 can't / couldn't have + V3。不要用 must be 來說過去的事。詳見“情態動詞 + have + V3”一課。

• The ground is wet. It must have rained last night.

  地是溼的，昨晚一定下過雨。

• He can't have finished already — he only started ten minutes ago.

  他不可能已經做完了——他十分鐘前才開始。

• ✗That mustn't be true.

  ✓That can't be true.

  Mustn't 的意思是“不允許”；否定的推測用 can't。

• ✗He must is at home.

  ✓He must be at home.

  must 後面用動詞原形。

• ✗She must to be at work.

  ✓She must be at work.

  must 後面不加 to。

• ✗You can't being serious.

  ✓You can't be serious.

  can't 後面用原形 be。

• ✗He must works here.

  ✓He must work here.

  must 後面不加 -s。

• ✗Lan didn't come yesterday. She must be sick.

  ✓Lan didn't come yesterday. She must have been sick.

  推測過去的事，用 must have + V3。