(Not) as far as I know
We use this expression to say that we think we know something, but we are not completely sure.
当我们认为自己知道某事，但又不完全确定时，我们可以用这个表达。
As far as I know, there is not such a rule in English grammar.
据我所知，英文语法中没有这样一条规定。
A: Have we got a test tomorrow?
我们明天有考试吗?
B: Not as far as I know. I think it's next week.
据我所知没有。我记得是下周。