有读者问： 有这样一句话：
(1)Mary shouldn't have received my letter; otherwise she would have replied before now.
这里为什么不用couldn't have received而用shouldn't have received?
should在此亦表“可能”，但不似could那么肯定。“should have+过去分词”表“可能”或“大概”的情况并不罕见，除上述句（1）外，再如：
(2)John should have left by tomorrow.
(3)Have you heard from Maria? She should have started her job on Monday.
(4)Lest you should not have heard all, I shall begin at the beginning.
顺便说一下，“should have+过去分词”还可以表“竟然……”和“应该……”等，例如：
(5)It is strange that he should have done it.
(6)The task should have been completed before the meeting.
