He has finished his homework when his mom went home.那为什么说when 不可以和现在完成时用？

He has finished his homework when his mom went home.
那为什么说when 不可以和现在完成时用？

是的,把has 改成had.发生在过去的过去的动作要用过去完成时

当他妈妈到家的时候他已经做完作业了
had

把has改成had ,因为从句部分已经是过去式了，主句要用过去完成时