I don't know whether you happen to have heard,but I'm going to study in the USA this September.我一直在纠结一个问题 为什么这里要说you happen to have heard 而不是 you have happened to hear?

I don't know whether you happen to have heard,but I'm going to study in the USA this September.我一直在纠结一个问题 为什么这里要说you happen to have heard 而不是 you have happened to hear?
I don't know whether you happen to have heard,but I'm going to study in the USA this September.
我一直在纠结一个问题 为什么这里要说you happen to have heard 而不是 you have happened to hear?

I don't know whether you happen to have heard,but I'm going to study in the USA this September.我一直在纠结一个问题 为什么这里要说you happen to have heard 而不是 you have happened to hear?
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.

是碰巧已经听说，不是已经碰巧听说，所以have heard 是hear 的完成时，而不是happen的完成时