现在完成时和过去完成时的区别就在于一个是现在一个是过去
现在完成是以现在的状态描述一个现在完成的事情,而过去完成则是以过去的状态,描述一个过去的过去完成的事情
比如：I had an exam yesterday,I had studied about it.
我昨天有一个考试,我已经完成了复习（考试发生在过去,而复习则是发生在考试之前,过去的过去）
I will have an exam tomorrow,I have studied about it.
我明天有一个考试,我已经完成了复习（考试在明天,而复习则是今天完成的）
will have done和will have been doing的区别则在于一个突出将来要完成的事情,一个突出过去在做,将来还要继续做的事情
比如：I will have studied about my exam tomorrow.
我明天就会完成我对考试的复习
By tomorrow,I will have been studying about my exam for 5 days.
到明天,我为了考试准备复习就已经5天了
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