太郎が今ヨーロッパに行っている。 ‘Taro is now in Europe (as a result of having gone there).’
太郎が去年ヨーロッパに行っている。 ‘Taro is the one who has the experience of having gone to Europe last year.’
Can the latter mean the same as the former saying he went to Europe and has stayed there since?
What makes the latter different in meaning while both say ヨーロッパに行っている? Is it because of 今 and 去年？
What if both sentences have no indication of when it happened? That is 太郎がヨーロッパに行っている? What could this mean?
Let me use は instead of exhaustive-listing-が to make things simpler.
太郎は去年ヨーロッパに行っている。
(Literally: "Taro has travelled to Europe last year.")
Basically you cannot tell where Taro is now only from this sentence. He may be still in Europe now, or he may have already returned to Japan, or he may be in Africa now. The interpretation purely depends on the context. If the previous context is "Where is Taro now?", then this sentence means he is still in Europe now. If the previous context is "Has anybody ever been to Europe?", then this sentence is about his experience in the past.
On the other hand, the meaning of 太郎は今ヨーロッパに行っている also depends on the context. It can mean either "He is in Europe now" or "He is on his way to Europe now".
