Question

he would have finished his college education,but he had to quit and find a job to support family

句中为什么用had不用would have

Answer 1

这句话涉及虚拟语气用法。过去时间段的虚拟语气，句式 ：主句谓语 would/should/could/ have +过去分词 ，从句谓语 had+过去分词。 而从句有时候是可以省略的。
这句话补充完整应是 If he had money,he would have finished his college education,but he had to quit and find a job to support family.
注意 but 转折后跟的是 had to do 用法，而不是had done哦。换言之 but 是过去式时态，与虚拟语气无关。
这句话可以这样理解：（如果有足够的钱）他本应该完成学业，但现在不得不退学找工作赚钱养家。

Answer 2

因为这句是虚拟语气，意思是“他原本可以完成他的大学学业”，其实并没有完成，后半句就是这个意思。