Translation:Which countries have you gone to?
Absolutely horrible English. Yes Which countries have you been to? is correct and the English sentence should be corrected.
Well, technically it should be "to which countries have you been" so that it doesnt end on a proposition but I won't tell.
There's actually nothing technical about that rule. It's an arbitrary one made up on the basis that English should emulate Latin grammar.
Not really, Japanese people use kanji a lot, it's just that using 事 would be incorrect in this case, as こと is a particle
In short, こと is "thing." It's basically the intangible version of 物. So in this case it effectively makes the concept of "having gone to a country" a noun.
In English, this question is in the present perfect tense, have + past participle of the verb. Japanese does not have a present perfect tense. The equivalent of asking a person if they have the experience of doing something is
(past tense of the verb) koto ga arimasu ka?
itta koto ga arimasu ka?
Have you been to ~ ?
I think 何カ国に行ったことがありますか (nankakoku ni itta koto ga arimasu) would work.
Yes, while this is correct English, many people often don't speak this way. I only use this sentence structure when writing.
Why is it not "どの国に行ったのがありますか"?? That was the form we used a lot in the sections before
Because this construction requires こと. -たことがある is a set construction expressing what is technically referred to as the "experiential aspect", that is, a form indicating that one has had the experience of doing something.
Wow, John Dryden has a Duolingo account? When you're not sure about the idiomatic construction, you can use the capitalisation as a hint.
"To which countries have you gone" was marked wrong. Duo likes dangling infinitives.