I thought I'd understood the differences until I actually had to use them more often to express myself better. Since this is an old, recurring grammar topic, I'll just lay out all possible forms I can think of below.
僕は--
勉強してる:
I am studying (as of now)
勉強していた:
I had been studying / had studied (until a certain point in time in the past)
勉強してくる:
I have studied / have been studying (until now, and will continue to do so until a certain point in the future)
勉強してきた:
I have studied / have been studying (until now)
勉強してきている:
I have been studying (until now, whether I will continue or not is not specified)
勉強してきていた:
I had been studying (until a certain point in time in the past)
Problems:
- If my understanding is correct, some of them seem to overlap.
- Do Japanese people nitpick about the different usages laid out above? (Regardless of how conscious this process is.)
どうぞよろしくお願いいたします。
user4092 My mistake. I meant to say 殺してきた and 殺してきている。I'm really sorry for the blunder.
Incidentally, my language exchange partner kept correcting my use of てくる and the works today, so it is definitely something that registers.
– Yeti Ape Sep 22 '18 at 12:31