he said he would come to my home in a week .为什么是填would come .有什么语法么

问题描述:

he said he would come to my home in a week .为什么是填would come .有什么语法么
还有选项是 came comes will come

he says he will come to my home in a week(in a week 一个星期后,要用将来式)
但题目是 he said ,will come 就要相应的改成 would come(would是will的过去式)