sure前面用什么介词

前面用介词for，for sure意思是确定，一定，例如：

1、One thing is for sure ─ it's not going to be easy.有一点可以肯定——事情不会很容易。

2、No one knows for sure what happened.谁也不知道究竟发生了什么事。

3、I think he'll be back on Monday， but I can't say for sure.我想他星期一会回来，不过我不敢肯定。