But the one thing we know for sure中的for是用作介词?

问题描述:

But the one thing we know for sure中的for是用作介词?
我查了一下 know for sure 好像是固定搭配,这个固定搭配中,know是名词做主词+for+sure做定语?

for sure 是副词 句中做状语 adv.确实地;毫无疑问地
Know 不是跟for sure 搭配,know 的宾语是前面的one thing,正常的语序是 but we know one thing for sure...