Can anyone explain the syntax of "I saw it happen"?

[Starboard Tack]

12. Well, I come from the school of traditional grammar

I haven't delved too deeply into Chomskyan theory on transformational grammar.

I must admit that I had my doubts about this, and still do to a degree. The subjunctive mood has become fairly redundant in English, unfortunately. Few people seem to use it in contemporary English, and those who do are seldom conscious of doing so, let alone explaining it.
My familiarity with the subjunctive comes mainly from my knowledge of romance languages, which actually gave me reason to pause in this case.
"I saw it happen" translates into all romance languages with "happen" being used in the infinitive, rather than the subjunctive.

For example:
French: "j'ai vu ça se passer" or "je l'ai vu se passer"
Italian: "l'ho visto succedere"
Spanish:"lo vi pasar"

"I saw him eat" translates as "L'ho visto mangiare" (Italian) or Je l'ai vu manger (French), or "lo vi comer" (Spanish)

So, I don't want to commit to my previous answer, yet. I did email an English professor who confirmed my original determination that it was "past subjunctive", but I'd like a second or third opinion on that.