问题
选择题
I coutdn't have got to the meeting on time____________ an earlier train.
A．if I had not caught
B．unless I had caught
C．if ! did not catch
D．unless I caught
答案
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：1 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。