Is が wrong in sentences like 私がplaceに行ったことがある?

Question

I saw someone getting corrected for using が in a ことがある sentence, and I was wondering why using が in sentences specifically like 私がplaceに行ったことがある is wrong, I can't seem to wrap my head around it.

I was thinking of some things like 私があそこに行ったこと which seem to be correct (in my mind). But I'm confused as to why it was considered wrong?

Edit: Changing が with の seems to reveal how it's wrong, I think because the は that is used in the relative clause is the topic and not the subject that does the verb, is this understanding correct in any way?

Answer 1

私は日本に行ったことがある。
I have been to Japan.

In this sentence, 私 is the topic of the entire sentence, and it's not in any relative clause. (Very literally: "As for me, a having-gone-to-Japan [fact] exists.") Structurally, this is yet another "double-subject" sentence (Aは + Bが + predicate) such as 彼は背が高い, 今日は仕事がある or このレストランは料理が美味しい. Naturally, 私 must be marked with は.

If が is used instead of は, it would work as an exhaustive-listing が ("It's me who has been to Japan"). This is still grammatical but makes sense only in a special context.

When you wrap this entire sentence in a subordinate clause...

彼は[私が日本に行ったことがあること]を知らない。
He does not know [that I have been to Japan].

Now, you must say 私が because 私 is in a subordinate noun clause (marked with the square brackets) and the topic of the main (outermost) clause is 彼.