I'm telling you

(also I tell you, I can tell you)

used to emphasize that what you are saying is true, and should be believed

(用来强调所说的是真的)听我说，我说的没错

I'm telling you, he's the best player in the American League.

我跟你说，他是美国联盟中最好的队员。

We’ve been waiting a long time for this, I’ll tell you.

我告诉你，我们等这个已经等了很久了。