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Which one is correct?
Yesterday Jack borrowed my bike and said he would return it very soon. and Yesterday Jack borrowed my bike and said he would return it very quickly.
I think both of them are correct, but my teacher said very soon is correct, because soon describes the duration of the action from borrow to return, whereas quickly talks about the action, return, itself.
Both are correct and easily interpreted, and the overlap is near-total. But your teacher is right about the shades of meaning: "quickly" faintly suggests that he's literally quick about borrowing, using, and returning the bike, whereas "soon" only references the time and says nothing of the manner.
This does not mean we wouldn't say either one just as readily. We would. But the mental picture would be slightly different.
Note that in past tense we might be more inclined to say "quickly", since "soon" is not available and the alternatives are a little more marked: "soon after that" or "shortly afterwards" or "before long".
Also, in colloquial speech, I'd usually say "fast" instead of "quickly" — but it would otherwise fill the same role described above.
Both are correct, and your teacher is right; quickly means the act of returning it will be done in a quick way, whereas soon means the act will be done after a small amount of time.
