only when you get stuck in a difficultsituation,you find who is your true friend

只有当你陷入困境中,你才能发现谁是你真正的朋友.
Only when you get stuck in a difficult situation can you find who is your true friend.为什么啊！句子中间有逗号还是一个句子吗？因为only+状语后面要用部分倒装。原句是：You can find who is your true friend only when you get stuck in adifficult situation.能不能用could代替can？最好不要。习惯上用can.时态上是过去时态时用could。