He doesn't know the answer,so he doesn't put up his hand(改为虚拟语气)

问题描述:

He doesn't know the answer,so he doesn't put up his hand(改为虚拟语气)
If he ______ the answer,he______ _______ _______ his hand.

If he __knows____ the answer,he__would____ __put_____ ___up____ his hand.为什么第一个空填的是knows 而后面是would? 时态貌似不符。我想填 knowswillputup是否可以?