助動詞：can / may / must / should / have toModal Verbs

助動詞後面一定接原形動詞

can、could、may、must、should 這些助動詞放在動詞前面，幫動詞加上「會、可以、必須、應該」的意思。它們有兩個規矩：後面的動詞一律用原形；主詞是 he / she 也不加 -s（不能說 cans、musts）。否定直接在助動詞後面加 not，問句把助動詞搬到句首。

• My little brother can swim.我弟弟會游泳。

• You should drink more water.你應該多喝水。

• She can't come to the party.她不能來派對。

• Can you help me with this box?你可以幫我搬這個箱子嗎？

常見助動詞的意思

否定縮寫：can't、couldn't、mustn't、shouldn't、don't / doesn't have to。

have to 要跟著主詞和時態變

have to 的意思跟 must 很像，但它其實是一般動詞：主詞是 he / she / it 要用 has to，過去用 had to，否定和問句要借 do / does / did。must 沒有過去式，說「以前必須」要用 had to。

• Amy has to take care of her sister after school.Amy 放學後得照顧她妹妹。

• We had to wait for the bus for thirty minutes.我們那時得等公車等三十分鐘。

• Does he have to wear a uniform?他需要穿制服嗎？

must not 和 don't have to 意思差很多

中文「不必」和「不可以」差很多，英文也是。must not（mustn't）是「絕對不可以」，是禁止；don't have to 是「不需要、不必」，做了也沒關係。考試最愛考這一組。

• You must not eat or drink on the MRT.在捷運上不可以吃東西、喝飲料。

• Tomorrow is Saturday. We don't have to go to school.明天是星期六，我們不用上學。

• She doesn't have to cook tonight. They're eating out.她今晚不用煮飯，他們要去外面吃。

常見錯誤

• ✗He can plays the piano.
  ✓He can play the piano.助動詞後面接原形動詞，不加 -s。
• ✗She musts finish her homework.
  ✓She must finish her homework.助動詞本身不會因為主詞是 she 而加 -s。
• ✗You should to see a doctor.
  ✓You should see a doctor.should 後面直接接原形，不加 to；只有 have to 才有 to。
• ✗He have to clean his room.
  ✓He has to clean his room.have to 要跟主詞變化，he 用 has to。
• ✗You must not bring lunch. We will buy pizza for everyone.
  ✓You don't have to bring lunch. We will buy pizza for everyone.意思是「不必帶」，要用 don't have to；must not 是「不准帶」。