为什么I had worked in a car factory for two years before I came here.用现在完成时而The famous scientists has returned home.He stay
问题描述：
为什么I had worked in a car factory for two years before I came here.用现在完成时而The famous scientists has returned home.He stayed in Africa for 13 years.后半句不用过去完成时?
答
1.I had worked in a car factory for two years before I came here
句意是：我在来这儿之前已经在一家汽车工厂工作了两年.
本句中before 引导的时间状语从句是一般过去时,“我在一家汽车工厂工作了两年”是发生在“来这儿之前”,即“过去的过去”,所以用过去完成时.
2.The famous scientist has returned home.He stayed in Africa for 13 years.
本句句意是：这位著名的科学家已经回家了.他在非洲待了13年.
句子只是强调“他在非洲待了13年”这一动作.如果两句合为一句,动词形式就需要变化了.
The famous scientist had stayed in Africa for 13 years before he returned home.
意思是：这位著名的科学家回家之前在非洲待了13年.
即“在非洲待了13年”发生在“回家（过去的动作）”之前,属于过去的过去,这时需要用过去完成时.