A warning is something that makes us aware of possible danger. There are a number of ways of warning someone. Warnings can be weak or strong.
Weak warnings are similar to advice. We can use a number of phrases:
I wouldn’t swim there if I were you.
I don’t think you should drink the water here.
I don’t think you ought to say no to the job offer. (more formal)
A word of warning, there are snakes on the trails. (Be careful)
We usually use don’t in strong warnings:
Don’t cross the road when the pedestrian light is red.
Don’t try to open the door when the train is moving.
Whatever you do, don’t take a lift from an unofficial taxi driver.
Sometimes we say I warn you or I must warn you:
It’s a wonderful restaurant but, I warn you, it isn’t cheap.
I must warn you it gets very cold at night in the mountains. Make sure to bring some warm clothes.
We often use beware of, caution, warning and danger in public warning notices:
Beware of the dog.
Caution: Wet floor.
Warning: this building site is private property.
[near a river where it is dangerous to swim]
Danger: Strong currents.
When we want to say something to warn someone about an immediate danger, we can use phrases like careful, be careful, watch out, look out, mind:
Be careful! That chair is broken!
Watch out! You’re about to hit the car behind you.
Look out! There’s a rock falling.
Mind your step! (pay attention to the step)
