Written by a native speaker?

Seems like a strange question. If you ask me, offhand, all the compositions which are in parentheses look fine, while just about all the "alternates" are slightly off.

How did you come up with these alternates? Are you just "experimenting?"

Oh yes, I should have first explained why I was asking this question. Sorry!
Someone wrote the above and I wanted to run it by the English experts here.
I'm studying English and there is this online English language forum I check out sometimes. It's a Korean site so all the questions are answered by Koreans. They're not native English speakers (but their English is fine I think). There is this one guy though who insists English is his first language. I had my doubts but as a non-native myself, I just couldn't substantiate my skepticism. Then I've just found this site. All the sentences were written by that person. What's in the parenthesis is just my interpretation.

Heh. Your understanding of English is 20 times better than his. The guy's a damn fraud, without question.

PS: I was just trying to be polite when I said "slightly off." For any true native speaker, they would be COMPLETELY off.

Oh my! Thanks so much for the clarification!