推断：must / can't

根据证据进行推测时，如果你确定某事是真的，用 must；如果确定它不是真的，用 can't。介于两者之间的是 might / may / could（有可能）。它们后面都接动词原形。

• must / can't + be + 形容词、名词、地点：He must be at work。
• must / can't + 动词原形（常是 know、have、live、like…）：She must know him。
• must / can't + be + -ing 表示正在进行的事：They must be watching the match。

💡只有证据充分、你几乎可以肯定时才用 must。把握不大的猜测，用 might、may 或 probably。

根据你看到、听到或知道的情况推断出结论时，用 must。证据常常就在同一句或下一句里。

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

  你站了一整天，一定累坏了。

• The lights are off. They must be out.

  灯都关着，他们肯定出去了。

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

  阿明一定知道答案——他是数学老师。

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

  听那动静，邻居肯定在开派对。

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

  你一定是阿兰的哥哥吧，你们长得太像了！

推测时，must 的否定是 can't（或 couldn't），不是 mustn't——mustn't 的意思是“不允许”。美式英语中人们也说 must not（不缩写）表示“我推断这不是真的”：He must not be home——但 can't 到哪儿都通用，是最稳妥的选择。

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

  那不可能是阿俊，他这周在新加坡。

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

  十美元买台新笔记本电脑？不可能吧。

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

  她不可能饿——她刚吃了一顿丰盛的午饭。

• You can't be serious!

  你不是在开玩笑吧！（表示惊讶的固定说法）

💡这里的 “can't” 不是说能力（I can't swim）；它的意思是“这不可能 / 不是真的”。

同一个词，两种意思。表示推测时通常接 be、be + -ing 或状态动词（know、have、like、live）；表示规定时通常接动作动词——但一定要看上下文。另见“义务：must / have to / need to”一课。

推测已经发生的事，用 must have + V3 和 can't / couldn't have + V3。不要用 must be 来说过去的事。详见“情态动词 + have + V3”一课。

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

  地是湿的，昨晚一定下过雨。

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

  他不可能已经做完了——他十分钟前才开始。

• ✗That mustn't be true.

  ✓That can't be true.

  Mustn't 的意思是“不允许”；否定的推测用 can't。

• ✗He must is at home.

  ✓He must be at home.

  must 后面用动词原形。

• ✗She must to be at work.

  ✓She must be at work.

  must 后面不加 to。

• ✗You can't being serious.

  ✓You can't be serious.

  can't 后面用原形 be。

• ✗He must works here.

  ✓He must work here.

  must 后面不加 -s。

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

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

  推测过去的事，用 must have + V3。