He is very grateful to the girl who helped him.后面的时态如果用完成时态是用had helped还是has helped

题目

He is very grateful to the girl who helped him.后面的时态如果用完成时态是用had helped还是has helped
还有这个句子里面They were surprised at the scenery and children that they saw （他们所见到的）down the river. 可以用that they have seen 吗,还是用 that they had seen? 我总是分不清楚用哪种时态,求详解……

答案

第一：要是用完成时态的话只能用has helped,用had的话前面的is改为was就可以了
第二：可以用had seen
have done是现在完成时,表示一个动作从过去发生一直持续到现在,到不了现在的就不能用现在完成时；had done是过去完成时,表示的动作是在一般过去式之前发生的,所谓“过去的过去”,如果一个句子没有过去式一般来说很少用过去完成时（虚拟语气有时例外）