by,搭配完成时


【问】

By about 6000 BC, people ____ (discover) the best crops to grow and animals to raise.

答案:had discovered

问题:为什么用过去完成时?用一般过去时discovered可以吗?

【答】

句意:到公元前6000年,人们已经发现了可以种植的最好的植物和可以饲养的最好的动物。

句中by的含义是“到……时为止”,这个含义通常搭配各种完成时。

再如:

  • By the time I got to the station, the train had already gone. (搭配过去完成时)
  • By next year, we will have learned 3,000 words.(搭配将来完成时)
  • By now, we have learned 3,000 words. (搭配现在完成时)