Be on your guard.

你要提防着点儿。

No one knows for certain what happened, but it's a fair bet that it was not an accident.

没有人知道究竟发生了什么，但几乎可以肯定这并非意外。

Leave well enough alone.

得过且过，见好就收。

Do you know that I mean?

知道我的意思吗？

That's not the issue.

那不是我们讨论的话题。

You've got the facts wrong.

你把事实搞错了。