1could主要指过去时间,表示能力,
例：Two eyes can see more than one.两只眼比一只眼看得清.
Could the girl read before she went to school?这女孩上学前能识字吗?
2.should
1）表义务.意为“应该”（某件事宜于做）,用于各种人称.
You should be polite to your teachers.你对老师应该有礼貌.
You shouldn´t waste any time.你不应该浪费时间.
2）表推测,意为“想必一定、照说应该、估计”等.
The film should be very good as it is starring first—class actors.
这部新电影是一流演员主演的,估计拍得很好.
They should be home by now.照说他们现在应当已经到家了.
