been in: 在

He's been in the doldrums ever since she left him.

自从她离开他以来，他一直很消沉。

been to: 去过，到过

I've never been to the States.

我从未去过美国。

been at: 在

We found out later that we had been at the same school.

后来我们才弄清楚我们是校友。