warn somebody about something
warn somebody against something
warn somebody of something

You should have been warning us about her, not the other way around.

They could have warned him about this before he came down.

He's warning about the fire next time, which, according to the movie, is here and now.

He had tried to warn her last night about the 17 change in plans.

I heard him warning his people down there about you.

The only thing he could really do was to warn the others about the change in the situation.

But I would like to warn you about one thing.

At the time, though, no one warned him about the risk of heart disease.

At the same time I had a growing feeling that someone was trying to warn me about something.

I mean, they've gone out of their way to warn me about this guy.

I'm the one you've probably been warned about, and you should believe every word.

It seems to me that you are the only thing I need be warned about.

Because it began to have the effects he had warned about.

A battle in progress, that he was too late to warn about?

"I don't think there's a consensus of what patients should be warned about."

It means people could actually be at more risk of what they are being warned about.

"This is what I have been warning about," he said of the losses.

The global financial pandemic that I and others had warned about is now upon us.

Well I don't think it actually needs warning about.

Suppose the creatures were actually a clear and present danger that the world really needed to be warned about.