Mary my letter; otherwise she would have replied before now.
[A] couldn’t have received [B] ought to have received
[C] has received [D] shouldn’t have received
(答案为A)
3)may / might have +过去分词，表示对已发生的事情做不肯定、可能性很小的推测，或事实上根本没发生，译为“也许……”。如：
At Florida Power’s Crystal River plant, a potentially serious leakage of radioactive water may have been unknowingly caused by an electrician.
2.表示虚拟语气。
1) needn’t have + 过去分词，表示做了不必做的事，相当于”didn’t need to do”,译为“其实没必要……”。如：
You needn’t have come over yourself.
As it turned out to be a small house party, we so formally.
[A] needn’t dress up [B]did not need have dressed up
[C] did not need dress up [D] needn’t have dressed up
(没有必要穿的那么正式，体现是说话者的建议，实际结果是否真的穿的很正式没有确定，答案为D)
2)should have +过去分词，表示应该做某事但实际上未做，译为“本应该……”should not + have过去分词表示本不应该做某事但实际上做了，译为“本不应该……”。如：
I regret having left the work unfinished; I should have planned everything ahead carefully.
我本来应该事先认真地把每件事情规划的很好，但实际上作者还是没有规划好，以至工作没有完成。
3) ought to have +过去分词，表示动作按理该发生了，但实际上未发生，译为“该……”，与should 的完成式含义类似。如：
The porter ought to have called the fire-brigade as soon as he saw the fire in the stock, which went up in smoke .
4) could have +过去分词，表示过去本来可以做但却未做，译为“完全可以……”。这点与ought/should/ have +过去分词用法相似。如：
What you said is right, but you could have phrased it more tactfully.
5) may/ might have +过去分词，表示过去可以做但实际未做，译为“(那样)也许会……”。如：
It might have been better to include more punchy statistics and photos of equipment in the introduction to further assist first-time office automation managers.
二.几个情态动词常考的句型：
1).may/might (just) as well “不妨，最好”，与had better相近;
Since the flight was cancelled, you might as well go by train.
Smoking is so harmful to personal health that it kills people each year than automobile accidents.
[A] seven more times [B] seven times more
[C] over seven times [D] seven times
(答案为B)
“Do you regret paying five hundred dollars for the painting?”“No, I would gladly have paid for it.”
[A] twice so much [B] twice as much
[C] as much twice [D] so much twice
(答案为B)
My uncle is as old again as I am
4)下列词和短语不用比较级形式却表示比较概念：
inferior, minor, senior, prior, prefer to, superior, major, junior, preferable, differ from, compared with, in comparison with, different from, rather than. 如：
Their watch is to all the other watches on the market.
[A] superior [B] advantageous [C] super [D] beneficial
(答案为A)
Prior to his departure, he addressed a letter to his daughter. (1986年考研题)
5)“比较级+and +比较级”或“more and more/less and less +原级”以及“ever, steadily, daily等副词+比较级”结构表示“越来越……”的意思，与这类结构搭配的常用动词有grow, get ,become等。前面两种情况更多地出现在进行时态中，注意的是这三种情况引导的比较级后面都不需要用than.如：
Things are getting worse and worse.
As I spoke to him he became less and less angry.
Her health was becoming daily worse
The road got ever worse until there was no road at all = the road got worse and worse.
6)比较级前面可以用even, still, yet ,all the (more)等修饰语用以加强语气，表示“更加”的意思。如
Today it is even colder than yesterday
I have yet more exciting news for you
7) 有关比较级的特殊句型：
A): not so much…as…与其说……不如说……
The chief reason for the population growth isn’t so much a rise in birth rates a fall in death rates as a result of improvements in medical care.
The ideal listener stays both inside and outside the music at the moment it is played and enjoys it almost as much as the composer at the moment he composes.