It's two years since I worked in Beijing.为什么不能用has worked?He worked in Africa on wild animals for eight years.为什么不能用
问题描述：
It's two years since I worked in Beijing.为什么不能用has worked?
He worked in Africa on wild animals for eight years.为什么不能用had worked?
Our work not finished ,we refused his offer.为什么不能用has not been finished?
答
你的第一句最好加上last,这样你就明白了：
It's two years since I LAST worked in Beijing.---------不在那里工作已经两年了.
第二句只强调在过去,到底是怎样的过去无所谓,与“今天“没有任何关系,所以只用”一般过去时“.如果是had worked,句子应该是这样的：
He had worked in Africa on wild animals for eight years before he came to be our biology teacher last year.--------过去的过去,用”过去完成时“.
Our work not finished == as our work hadn't been finished yet -------你的是独立主格结构,我的是原因状语从句结构.-------独立主格结构需要去掉助动词had 和been,保留真正动词.