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.