Lol, jesus christ you didn’t even learn how to break down a sentence in elementary school but you think you can go around correcting people?

“before some random idiot...” is a prepositional phrase. AKA a modifier. Here is a quick grammar refresher.

http://www.chompchomp.com/terms/prepositionalphrase.htm

Remember that a prepositional phrase will never contain the subject of a sentence.

Sometimes a noun within the prepositional phrase seems the logical subject of a verb. Don't fall for that trick! You will never find a subject in a prepositional phrase.

Your sentence, with correct grammar, is

I was in this thread before some random idiot...

The slang term “inb4” or “in before” implies the subject.