动词不定式的完成式,表示在谓语动词之前发生的动作,比如：
To have known her is a privilege.
This house seems to have been built by a very good architect.
By 1992,he hoped to have opened a branch overseas.
There is no need to have bought a new pair of shoes for me.
He is too young to have seen the old society.
They might have found him to have lived longer,had a particular circumstance not occurred.
The game was originally scheduled to have taken place in Rome.
i intended to have finished my worked last night.
你给出的句子也是一样,意思是：“我不知道你听说没有,今年九月我要到美国去学习了.”happen表示“碰巧”时,一般不用现在完成时,并且“听说”在说话之前,“碰巧”在后,所以用动词不定式的完成式表示.希望我能帮助你解疑释惑.