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.
I am studying (as of now)
I had been studying / had studied (until a certain point in time in the past)
I have studied / have been studying (until now, and will continue to do so until a certain point in the future)
I have studied / have been studying (until now)
I have been studying (until now, whether I will continue or not is not specified)
I had been studying (until a certain point in time in the past)
- 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.)