He can ride a bike now, but he couldn't a few weeks ago.
他现在能骑自行车了，但几星期前他不会。
· A poor boy like me couldn't go to school.
象我这样的穷孩子上不了学。
· When my father was young he could run fast.
我父亲年轻时能跑得很快。
He said he couldn't arrive before 6 o'clock.
他说他不能在六点钟前到达。
It could rain tomorrow
If I were a bird, I could fly in the sky.
假若我是一只鸟，我就能在天空中飞翔。
They could have completed this work.
他们本来是能够完成这项工作的。
· Nothing could have saved him even if he had been tended without delay.
即使当时他得到了及时的救护，他的生命也无法挽救。
· If he had been with you, you could have been successful.
假如他以前和你在一起的话，你早就能成功了。
· You could have caught the train if you had taken a taxi.
如果你租一辆出租汽车的话，你本来是可以赶上火车的
参考资料：http://zhidao.baidu.com/question/10039273.html