回复：请教一个学钢琴的理论方面问题：

Your situation is pretty clear. Set the metronome at M.M. 100, then match half note, which is one beat in 2/2 and two beats in 4/4 to each beat the metronome gives. That answers your 2nd question. However, you did not specify the piece you are talking about is in 2/2 or 4/4, thought you mentioned that adjust the metronome to 4 beats. If the piece is in 2/2, you should set the metronome to two beats instead of four. This refers back to your first question, what is the difference between 2/2 and 4/4. I am too busy to make a comprehensible answer now. I will come back to you on that one.