有读者问： 我在张汉熙主编的《高级英语》（上册）的第一课中看见这样一个句子：
(1)Yet Beethoven cannot have been an easy person to help, for he was quick to show resentment of patronage and social distinction...
请问为什么用现在式cannot而不用过去式could not? 又to help是否用作person的定语？
表示过去不可能发生的事，可以用“cannot+have+过去分词”，也可以用“could not+have+过去分词”，但could not的语气较cannot为轻。请你千万注意，cannot虽然是现在式，但如后接“have+过去分词”，即表示过去不会发生的事，再如：
(2)He can not have missed the way. I explained the route carefully and drew him a map.
(3)They can not have been easy to grow.
用“情态动词can+have+过去分词”表过去动作或状态时，除用于否定结构外，亦可用于疑问句 ，如：
(4)Can she have done such a thing?
(5)Can he have left already?
但要留意，“can+have+过去分词”不可用于肯定结构，例如不可说“He can have gone yesterday.” 但可以说：
(6)He could have gone yesterday.
也可以说：
(7)He may have gone yesterday.
(8)He must have gone yesterday.
