关于时态的问题，GWD17-Q32

sprite112

Navigators have known for thousands of years that the ocean has variable currents, but it is only in the last half century that a reasonably clear picture has emerged of the patterns and causes of ocean currents.

A) a reasonably clear picture has emerged of the patterns and causes of ocean currents
B) a reasonably clear picture of the patterns of ocean currents and their causes have emerged
C) a reasonably clear picture emerged of ocean currents, their patterns and the causes of them
D) there have emerged a reasonably clear picture of the patterns of ocean currents and what caused them
E) there had emerged a reasonably clear picture of the patterns of ocean currents and their causes

答案是A，我的疑问是既然发生在“ in the last half century”，为什么要用完成时，不用过去时？

lulutingyu

啊，终于明白了，谢谢，所以下面这句也是这个原因才用完成时的吗？


All Y chromosomes in existence t ...
sprite112 发表于 2011-7-30 06:01

    is thought to have done中have done其实是表示往前推一个时态，来区分is thought to do，因为to后面一定要跟动词原型，所以要做如此区分，所以，have done并不是表示完成时。

lulutingyu

in last century和in the last century有区别。
in the last century是指到这一瞬间截止，往前倒数的一个世纪，即1911年1月27到2011年1月27.（当然对于大范围时间不必如此精确）
而就我们这个时间2011年1月27号来言，in last century就是指20世纪了.

sprite112

这个我知道，因为是强调句 it is ... that...  还原就是
in the last half century，a reasonably clear picture has emerged of the patterns and causes of ocean current

rockstonexS

但是不懂的是 but it is only last half century 里面为什么用IS而不用WAS呢？

sprite112

啊，终于明白了，谢谢，所以下面这句也是这个原因才用完成时的吗？


All Y chromosomes in existence today are descended from a single ancestor’s who is thought to have lived about 140,000 years ago.
A) a single ancestor’s who is thought to have lived
B) a single ancestor’s who was thought to live
C) a single ancestor whom it is thought had lived
D) the Y chromosome of a single ancestor who is thought to have lived
E) the Y chromosome of a single ancestor whom it was thought had lived

mop000

in the last century, lz要记住哦，是大段时间，要用现在完成时
用现在完成时的标志：
1. since
2. 大段时间如in the pat several years, in the last century, in a period of time
3. 出现has ever seen或者than ever before possible等字眼