Collocations for warn - verb

clearly, explicitly
bluntly, firmly
The chancellor bluntly warned the Cabinet to cut public spending or face higher taxes.

darkly, sternly
softly
‘Don't,’ he warned softly.

specifically
We were specifically warned against buying the house.

duly
Having been duly warned that I would get nowhere with my application, I went right ahead and applied anyway.

always, constantly, repeatedly
My mother constantly warned me not to go into teaching.

publicly

had better, have to, must, should
I must warn you that some of these animals are extremely dangerous.
I thought I should warn her about it.

try to
I did try to warn you.

fail to
She claimed doctors had failed to warn her of the risks involved.

about
No one had warned us about the unbearable heat.

against
We were warned against drinking the local water.

of
The report warns of the dangers of obesity.
They warned us of the risks involved.

be warned
You will get better—but be warned, it may be a long process.

Warn is used with these nouns as the subject:
expert, report, scientist, sign, study

Warn is used with these nouns as the object:
investor, voter