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Difference between '~koto ga aru' and '~koto de aru'?

de aru = the couplula and means "desu" or "da"
~koto de aru = it's some kind of fact

ga aru = there is
koto ga aru...refers to something that there is

That is the best I can do without context.

Well you have already understood the usage of "koto ga aru".
It means having had some experiences of something. This expression
is very close to present perfect of
English. "koto de aru" is completely different expression.
1. It is said that ~
ex. 走ることは健康に良いことである。
Hashiru Koto ha kenkou ni yoi koto de aru.
It is said that running is good for health.

I hope this post helps you.