No one knows for sure [=for certain] what happened.没人知道到底发生了什么。

That was a mistake for sure. = That for sure was a mistake.那肯定是个错误。

(US, informal) “Do you want to come to the party?” “For sure.” [=I certainly do]“你想来参加聚会吗？”“肯定啊。”

“We don't want that to happen again.” “That's for sure!” [=that is certainly true]“我们不希望再发生那样的事。”“肯定不会了。”