Could this also mean Have you been to Stockholm? or would that be Har du varit till Stockholm?
As a native English speaker, I find the use of "in" here very odd. I would always say "to Stockholm".
Have to disagree with you on this one. "Have you been in Stockholm?" asks where someone has physically been lately as opposed to "Have you been to Stockholm?" which asks whether you have ever travelled to Stockholm. They are expressed differently in English but clearly not in Swedish - sounds like we need to use context to work out which meaning is intended.
eg "I haven't seen you for a while. Have you been in Stockholm?"
"I hear you are travelling around Sweden. Have you been to Stockholm?".
Would "Have you visited Stockholm?" be an acceptable translation, or would it stray too far from the intent of the question?
I believe the Swedish for that would be "Har du besökt Stockholm?". I get what you mean though - both phrases have basically the same meaning, but I think Duolingo wanted to keep it separate so we can learn the differences in translation. I suppose it's up to the admins to decide :)
Which perfect tense are you talking about?
It's very similar to English. Present and past perfect are created with the auxiliary verb "ha" = "have". Eg: "Jag har lagat middag" = "I have made dinner" // "Jag hade lagat middag" = "I had made dinner"
Future perfect is created with the verbs "ha" and "komma". Eg: "Jag kommer (att) ha lagat middag, när..." = I will have made dinner, when....
Conditional perfect is created with "ha" and "skulle"(ska). Eg: Jag skulle ha lagat middag, om inte ... = I would have made dinner, if not...
I was refering to the present perfect tense and what I actually meant whether ha is the only auxillary verb used to construct it. For example German uses sein (to be) for conditions and movement and haben (to have) for everything else: Ich bin gegangen = I have gone; Ich habe gegessen = I have eaten. Italian is pretty similar and sometimes uses avere and sometimes essere.