this is for sure,中sure是副词,这从语法上怎么理解,没有介词加副词的搭配啊

for sure口语用法=without doubt
例如I think he lives there but i couldn't say for sure我想他是住在那儿的,但是我不敢肯定.
（查自牛津高阶英汉双解词典sure adj 第六条短语系列.