I have finished my homework now. 现在完成
I had finished my homework when he came.过去完成
我已经完成了,现在.
此时就用现在完成,过去发生的对现在有影响的动作.
I had finished my homework when he came.
他来了,是过去发生的事情,在他来之前我已经完成了,用过去完成时.