I don't know whether you happen to have heard,but I'm going

題目：

I don't know whether you happen to have heard,but I'm going to study in the USA this September.
我一直在糾結一個問題 爲什麼這裡要說you happen to have heard 而不是 you have happened to hear?

解答：

動詞不定式的完成式,表示在謂語動詞之前發生的動作,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.
你給出的句子也是一樣,意思是：「我不知道你聽說沒有,今年九月我要到美國去學習了.」happen表示「碰巧」時,一般不用現在完成時,並且「聽說」在說話之前,「碰巧」在後,所以用動詞不定式的完成式表示.希望我能幫助你解疑釋惑.
再問： 碰巧和聽說爲什麼有時間先後呢？應該是同時發生啊？
再答： 一般來說，happen表示「碰巧」是不用現在完成時的，但是「聽說」是說這句話之前就已經發生的，動詞不定式必須用完成式形式，希望我能幫助你解疑釋惑。
再問： 那是不是hear不是在跟happen比較，而是在跟don't know之間比較時間先後？ 這麼理解對嗎
再答： 對，可以這樣理解，希望我能幫助你解疑釋惑。