What meaning does the object marking particle(을) give in the sentence : "그가 중국에서 2년을 살았습니다."? If the sentence translates to "He lived in China for 2 years" in English, why is 을 used to specify the time duration while it is not the object?
And on an unrelated note what difference would it exactly make if it is "그는" instead of "그가" in this sentence?
-을 after time duration usually gives a sense that this time duration is a fresh topic that is being brought up. I don't consider this -을 to be object marker.
For instance, if someone asked you about how long you've been living in Korea, then you would usually say: 2년 살았어 or 2년동안 살았어. It's without -을 with time duration because the duration was the topic of the conversation when you answered. However, if it's you who wants to start talking about the time duration, you can say 내가 한국에서 2년을 살았어.
Or, you can also use it to make emphasis on the time duration. 내가 3년을 공부했어 can sound like you're emphasizing on how long you've studied it. Like in other languages, an emphasis can be used for various reasons in different contexts. This may mean that you've had up with it, or just boasting it etc..
감옥에서 2년을 지내다. 미국에서 2년을 보내다. These sound natural to me. 을 can be used to indicate the time duration.
그가 - pointing out 'he' is the one who lived in China for 2 years.
그는 - It can also mean that the speaker has been talking about another person who lived in a different situation or time, and then the speaker continues and says 'he' is living in China for 2 years.
"그가 중국에서 2년을 살았습니다."
In English, you can say either "He has lived in China for two years" (intransitive verb "live") or "He has spent two years in China" (transitive verb "spend"). 2년을 살다 is thus similar to "spend two years", both 살다 and "spend" (or "pass" or some other) functioning as a transitive verb. It is just looking at the fact slightly differently - it doesn't have to be an intransitive verb. Another example would be "working along/down the street" (길(을) 따라 걷다) and "walking the street" (길을 걷다) - you can use the same verb, "walk" and 걷다, either intransitively or transitively.
그는 vs 그가
은/는 the topic marker 1) suits the default, normal way of your mind looking at the world (i.e. you have internalized it), 2) contrasts the noun with other (implied or mentioned) case.
이/가 the subject marker 1) suits a specific, external "happenings" in the world (you weren't necessarily thinking about it in your mind), 2) when the noun is needed by the context (because someone asked you, or it is needed in a sub-clause providing facts for the main point).
So 그는 중국에서 2년을 살았습니다 is the more common and proper way to talk about this particular fact, since it is something you know as a solid fact.
그가 중국에서 2년을 살았습니다 is not used as much, but it can be said if someone asked "Who has lived in China for two years?", for example. In this case, it is an anecdotal fact required by the particular situation of someone asking you, not from your own desire to express it, so you use 가.
Think of those Korean verbs as having a optional preposition in them. They can be either transitive or intransitive. For example, "살다" translates to "to live (for);" when it is transitive, the object is the duration.
내가 법학을 삼 년을 공부했어!
...Wait, it works. ...Never mind. It's not an object. It seems that the "을" particle after a duration is not an object marker.
