義務：must / have to / need to

表示必須做某事有三種說法：must、have to、need to——後面都接動詞原形。Must 是情態動詞；have to 和 need to 的用法和普通動詞一樣。

💡Must 永遠不加 -s、不加 to、不用 do：She must go。Have to / need to 正好相反：She has to go。Does she need to go?

1. have to——因為規定、工作或客觀情況而必須（是事實，不是說話人的個人看法）

• I have to wear a uniform at work.

  我上班必須穿制服。

• Linh has to take two buses to get to school.

  阿玲上學得坐兩趟公交車。

• Do you have to work on Sundays?

  你星期天也得上班嗎？

2. must——說話人自己覺得必須：提醒自己，或者強烈推薦

• It's late. I must go.

  很晚了。我得走了。

• I must call my grandma this weekend.

  這個週末我一定要給奶奶打個電話。

• You must try the coconut coffee here — it's amazing!

  你一定要嚐嚐這裡的椰子咖啡——太好喝了！

3. must——書面規定、標識和說明

• All visitors must sign in at the front desk.

  所有訪客都必須在前臺登記。

• Passengers must keep their bags with them at all times.

  乘客必須隨時看管好自己的行李。

4. need to——為了某個目的或因為當前的情況而需要做某事

• My phone is almost dead. I need to charge it.

  我手機快沒電了。我得充一下電。

• Do we need to book a table, or can we just walk in?

  我們需要訂位嗎，還是直接去就行？

5. have got to——和 have to 一樣，在非正式口語中非常常見

• Sorry, I've got to go — my bus is leaving.

  抱歉，我得走了——我的公交車要開了。

💡在美式英語中，表示義務的 must 聽起來相當強硬、正式；美國人通常說 have to 或 have got to（gotta）。拿不準的時候，用 have to 幾乎總沒錯。

Must 只有一種形式，只能談論現在或將來。表示過去、用在 will / might 後面、用於現在完成時或放在另一個動詞後面時，要用 have to。

• You mustn't touch the paintings.

  請勿觸控畫作。

• You don't have to take your shoes off. Come in!

  你不用脫鞋。進來吧！

💡mustn't = 禁止；don't have to = 可做可不做。不要把兩者弄混。另見“無需：don't have to / needn't”一課。

• ✗You must to turn off your phone in class.

  ✓You must turn off your phone in class.

  must 後面不加 to。

• ✗She have to finish her homework.

  ✓She has to finish her homework.

  he/she/it → has to。

• ✗Last week I must work every evening.

  ✓Last week I had to work every evening.

  表示過去用 had to。

• ✗Do you must wear a helmet?

  ✓Do you have to wear a helmet?

  must 不和 do 連用；要問 Do you have to…?

• ✗We will must show our passports.

  ✓We will have to show our passports.

  將來：will have to。

• ✗She needs wear a uniform.

  ✓She needs to wear a uniform.

  need 作普通動詞 → need to + 動詞。