Question

I couldn't have got to the meeting on time ________ an earlier train．

A. if I had not caught
B. unless I had caught
C. if I did not catch
D. unless I caugh

Answer 1

答案B
If…not和unless有时也不能互相替换。unless不能用于“由于未发生B而发生A”的句子里，如：I will be quite斟ad if she does not come this evening。unless常用来引导一个谈及过去的事后的想法，unless从句跟着主句，通常用破折号而不是逗号将它与主句分开。