你好.in,for和under的使用由从句部分的谓语决定：
1 There are cases. In those cases she comes late.
IN ------ There are cases IN which she comes late
2 Tell me the reason. You came late FOR that reason
For ----- Tell me the reason FOR which you came late
3 There are circumstances in our life. The right thing to do may become the wrong thing to do UNDER those circumstances
UNDER -------There are circumstances in our life UNDER which the right thing to do may become the wrong one to do
4 祝你愉快.