[請益] must 和 had to 選哪個?

[chocomacha (chocomacha)]

這個句子是克漏字的一部分

By the time they started to feel better, it was already the sixth day and
they ___ the next day.

(A)must leave
(B)had to leave
(C)was to leave
(D)want to leave

答案：B

想問的是
為什麼答案不能選A

--
※ 發信站: 批踢踢實業坊(ptt.cc), 來自: 1.172.228.130
※ 文章網址: https://www.ptt.cc/bbs/Eng-Class/M.1546863037.A.F54.html
推 EVASUKA: must+原形不能用於過去
→ EVASUKA: had to或ought to可以倒退，must不可
→ chocomacha: 謝謝回答