现在完成时，直到现在的某时刻之前完成的事，对现在有影响；或者过去发生并持续到现在的。结构have/ has +过去分词 例：i have already finished my homework. i have had the car for 2 years.
过去完成时，在过去的某一时刻以前完成的事，常和before, after, when等连用。结构had +过去分词。The train had already left before i got there.
将来完成时，将来的某一时刻以前将要完成的事。例：by the end of this year 结构：will have +过去分词 By the end of this year i will have learnt 2000 words.
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