I saw Mr．Zhang just now．He couldn't have gone to Beijing．
b．may／might+have done一般只用于肯定句或否定句，不用于疑问句，表示对过去情况不肯定的
推测，意为\也许已经(没有)……\。might have done还可用来表示\本来可
能……，\但实际上没发生的事。如：
He may (might) have gone
home．
He may not have finished the work．
You should not have
climbed that hill．You might have killed yourself． 另外，might+ have done还可以表达\本来应该或可以做但没有做某事\之意，含有轻微的责备口吻。如：
You might have given him more help，though you were busy．
c．must+ have done只用于肯定句，表示对过去情况肯定的推测，意为\想必，一定做了某事\。如：
It must have rained last night，for the road was quite muddy.
注意：如果要表示对过去情况否定的推测，不能用mustn't，要用can't have done或couldn't have done来表示：\不可能做了某事\。如：
He can't have been to Beijing，for he knows too little about it．
d. needn't+ have done通常用于否定句或疑问句中，表示不必须做的事情却做了。如：
You needn't have told her the news, for I had told her earlier．
注意：didn't need to do表示\没必要做而实际上也没有做某事\。如：
