这里为什么要用不定式?

问题描述:

这里为什么要用不定式?
Many of them turned a deaf ear to his advice,even though they know it to be valuable.
"...know it to be..."这里为什么要用'to be'而不用‘was’?

用不定式来作宾语补足语.用was也可以,这样就是know后接了个宾语从句,省了个that ,…know (that )it was …