英语改错(初三)

ago---before
ago一般前面接确定的时间，不可用于完成时．
而before只是一般意义的以前，一般用于完成时．

以前从没人登上过月球．

No one had ever landed on the moon before.

No one had ever landed the moon before.
ago不能跟完成时，只能跟过去时。before可以跟完成时。

ago改成before

No one had landed the moon ago

ago不能跟完成时,ago改成before .