아/어 왔다 when is it used?
I came across this grammar when reading my text book. I never learned this as an explicit grammar point but can also not find it anywhere online (at least in the english internet)
In my specific case:
중요한 역할을 해 왔다
I understand it means something like "has played an important role".
But obviously i cant use this grammar when ever is use present perfect in english. So when exactly is this grammar point used?

Question

아/어 왔다 when is it used?
I  came across this grammar when reading my text book. I never learned this as an explicit grammar point but can also not find it anywhere online (at least in the english internet)
In my specific case:
중요한 역할을 해 왔다
I understand it means something like  "has played an important role".
But obviously i cant use this grammar when ever is use present perfect in english. So when exactly is this grammar point used?

Answer

V. + (어/아) 오다
=> "to do . . . and bring"
V. + (어/아) 가다
=> "to do . . . and take," "to progress/go . . . (ADV.)"
-어/아 오다 and -어/아 가다 are useful helping verbs to indicate the direction of the main verb. 
There is no one-word verb for "to take" or "to bring" in Korean, 
but their equivalent expressions are 가져 가다 "to have and go" and 가져 오다 "to have and come." 
Similarly, Korean has 사 오다 for "to buy and bring," 캐 오다 for "to dig up and bring," and so on.

잘 해 오다	to do well and bring	잘 해 가다 to make well and take
요약해 오다 to summarize and bring 요약해 가다	to summarize and take
가져 오다 to bring, to fetch 가져 가다 to take away
사 오다	to buy and bring 사 가다	to buy and take
물어 오다	to bring by mouth 물어 가다	to carry away by mouth
(of dog, cat, and so on)	
알아 오다	to bring back findings	알아 가다	to take back findings 
집어 오다 to pick up and bring	집어 가다	to pick up by	hand and take away
씻어 오다 to wash and bring 씻어 가다	to wash and take away
싸 오다 to bring wrapped	싸 가다	to take wrapped 
찾아 오다	to come to visit 찾아 가다 to go to visit
끌어 오다	to bring by dragging 끌어 가다	to drag away

For some verbs, -어/아 가다 indicates that the action or event is "progressing (well/slowly/fast)," 
as in 잘 되어 간다 "it is going well" or "it is coming to (an end)," and as in 일이 끝나 간다 "the job is coming to an end."

For some verbs, -어/아 오다 indicates that the action or event is "have been .. (since past)," 
as in 먹어 오다 "have been eating" and as in 일해 오다 "have been working". usually used in the form of "오고 있다","왔다" or modifier form like "온, 왔던"

그는 중요한 역할을 해 왔다.
He has played an important role.
그는 중요한 역할을 해 오고 있다
He has been playing an important role.

내가 오랫동안 살아 온 마을
The town where I have lived for a long time.
내가 오랫동안 살아 왔던 마을
The town where I had lived for a long time.

Also refer to https://www.italki.com/question/365005

Answer

-아/어 왔다 means you have been doing something over a period of time, similar to the present perfect continuous in English.

So 그는 중요한 역할을 해 왔다 = He has been playing an important role.

Here are examples of different present perfect cases.
1 나는 프로젝트를 끝냈다 = I have finished my project. (completion)
2 나는 프로젝트를 망쳐 버렸다 / 망치고 말았다 = I have botched up my project. (result - unintended in this case)
3 나는 6개월 동안 이 프로젝트를 (진행)해 왔다 = I've been working on this project for six months. (continuation)
4 나는 지금까지 프로젝트 다섯 개를 끝냈다 = I have finished five projects until now. (experience and/or completion)

-아/어 왔다 is not the only way to say it though.  We often use simpler expressions like 하고 있다 (am doing it - present continuous) or simply 했다 (did - past) if the meaning would still be clear.  For example 6개월 동안 이 프로젝트를 하고 있어요 is as natural and clear as #3 above because  6개월 동안 indicates it's about what went on (or will go on if future tense follows) for six months.

Answer

이것을 이해하는것은 상당히 어렵습니다.  
almost of korean can't understand  present perfect tense too..
왜냐하면.. 한국어에서 simple past  and present perfect, these two 를 문법으로 구분하지 않아요. 
분명히 simple past and present perfect가 존재함에도 불구하고 말이죠