Past tense: "happen to have" or "happened to have"?
Which is the proper (i.e. grammatically correct) response?
Alice: "The earth is flat, and the sky is green."
Bob: "The earth is round, and the sky is blue."
Alice: "Can you provide indisputable proof of these claims?"
Bob: "No. It's just something I happen to have observed."
Bob: "No. It's just something I happened to have observed."
Solution 1:
As Peter Shor has written, only I happen to have observed is correct. Whereas have observed is perfect, it describes knowledge due to a past experience—similar to the Present Perfect tense. The verb happen, acting as stative verb, tells that the speaker is referring to his present knowledge.
Compare:
I happen to know your father.
(I know your father.)I happen to have known your father.
(I used to know your father.)
happen here emphasizes the random nature of this circumstance and the irrelevance of its causes to the conscious efforts of the three persons involved.
Solution 2:
From the corpus of the Longman Dictionary, it seems the general past tense form is ‘happened to do’. Although I think ‘happen to have observed’ sounds better in your example, as it is not really describing a past case.
(A search led me here because of the title, so I do not think my answer is completely irrelevant.)