If you hadn't borrowed him your Pokemon Pearl,this case would not happen!
If you had not lent him your game card/PM Pearl, this tragedy(case) would have not happened. Noun clause: If-clause/conditional中, 有關unreal situation 的都會轉tense的~ lend及borrow的用法sy已經講了=w= 我將"this case" 改為this tragedy的原因是"lent him your game card/PM Pearl" 都算是一個case, pronoun reference會可能confuse別人~