1，过去完成时
表示到过去某个时间点时，已经做过某事
例如：到去年9月，我已经学习英语两年了。
By last September,I had learned English for 2 years.
2, 原来应该做某事而没有做
in case of waking up the baby
I would have grasped the opportunity without the least hesitation.
we would have not rent the house with so low price.
We hope that there are more opportunities to put the knowledge learned in the class into practice
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