现在完成时,表示动作发生在过去,并且一直持续到现在,或对现在产生一定得影响.例如,I have worked in this factory since 1949.(工作这一动作从1949一直持续到现在）
过去完成时,表示动作发生在过去的过去,通常有一个和其相对比的过去时间状语出现在句子中.
Before i met her,i had heard of her name.(I met her 发生在过去,i had heard of her name 发生在 I met her 之前,即过去的过去,因此用过去完成时)