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

And I'm trying to warn you of a possible problem.

They were all warned, of course, before being allowed to go on leave.

Was the old man trying to warn her of something?

All I wanted to do is warn you of the situation.

There was no way to warn them of these changes.

The young man's manager warned him of the important people here.

He had not been able to warn her of his visit.

From the 1950s until the 1970s they warned us again of a coming ice age.

It was almost as though he were trying to warn her of something.

But, like other officials, he warned of possible future attacks.