知识点：不定式的完成式、进行式和完成进行式：这三类在考研题中出现的频率很高，要特别注意不定式的这三种形式
1、如果不定式所表示的动作与主要谓语所表示的动作(状态)同时(或几乎同时)发生或发生在主要谓语之后，那么用不定式的一般形式
He received a warm welcome from the Australians and from his family who had flown there to meet him.
In 1967,he arrived back in England,where a quarter of a million people were waiting to welcome him.
After all,eighty was a special birthday,another decade lived or endured just as you chose to look at it.
Now suddenly she began to sob,holding herself in as if weeping were a disgrace.
2、如果不定式所表示的动作(状态)发生在主要谓语动词或特定的某时间之前,那么不定式就要用其完成式
She feels relaxed to have finished writing her thesis before the deadline.
例：
1)The Vikings are believed America.
[A]to have discovered
[B]in discovering
[C]to discover
[D]to have been discovered
2)The students were to at the auditorium before 1:30 pm,but the lecture was cancelled at the last minute.
[A]assembled
[B]have assembled
[C]assembling
[D]be assembled
3)He was to the new ambassador,but he fell ill.
[A]having telephoned
[B]have telephoned
[C]has telephoned
[D]telephoning
3、不定式进行式表示主要谓语动词所表示的动作(情况)发生时,不定式所表示的动作正在发生
When he came in,I happened to be doing my experiment in the lab.
Why do you stand here?You are supposed to be working in the workshop.
4、不定式完成进行式表示其动词所代表的动作，在主要谓语动词所代表的动作之前一直在进行
The students from the Department of Sociology are said to have been investigating the possibility of producing the new product.
They are quite happy to have been cooperating harmoniously with us till now.
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