There are several ways of warning someone not to do something.
In conversation, you can say `I wouldn't ... if I were you'.
I wouldn't drink that if I were you.
A weaker way of warning is to say `I don't think you should...' or `I don't think you ought to...'.
I don't think you should try to make a decision when you are so tired.
I don't think you ought to turn me down quite so quickly, before you know a bit more about it.
You can also warn someone indirectly not to do something by saying what will happen if they do it.
You'll fall down and hurt yourself if you're not careful.
You can warn someone not to do something by accident or because of carelessness by saying `Be careful not to...' or `Take care not to...'.
Be careful not to keep the flame in one place too long, or the metal will be distorted.
Well, take care not to get arrested.
`Don't' is used in strong warnings.
Don't put more things in the washing machine than it will wash.
Don't open the door for anyone.
You can emphasize don't with whatever you do.
Whatever you do don't overcrowd your greenhouse.
Don't get in touch with your wife, whatever you do.
You can mention the consequences of not doing what you say by adding or and another clause.
Don't say another word or I'll leave.
People sometimes say `I warn you' or `I'm warning you' when warning someone, especially when preparing them for something they are going to experience.
I warn you it's going to be expensive.
I must warn you that I have advised my client not to say another word.
It'll be very hot, I'm warning you.
These expressions are also used as threats.
I'm warning you, if you do that again there'll be trouble.
More formal warnings
Never is used with an imperative in more formal warnings.
Never put antique china into a dishwasher.
Even if you are desperate to get married, never let it show.
`Beware of...' is used to warn against doing something, or to warn about something that might be dangerous or unsatisfactory.
Beware of becoming too complacent.
I would beware of companies which depend on one product.
The expression `A word of warning' is sometimes used to introduce a warning. So are `Warning' and `Caution', in books and articles.
A word of warning: Don't have your appliances connected by anyone who is not a specialist.
Warning! Keep all these liquids away from children.
Caution. Keep the shoulders well down when doing this exercise.
Warnings on products and notices
`Warning' and `Caution' are also used on products and notices. `Danger' and `Beware of...' are used on notices.
Warning: Smoking can seriously damage your health.
CAUTION: This helmet provides limited protection.
DANGER – RIVER.
Beware of Falling Tiles.
When you want to warn someone about something that they might be just about to do, you say `Careful' or `Be careful', or, more informally, `Watch it'.
Careful! You'll break it.
He sat down on the bridge and dangled his legs. `Be careful, Tim.'
Watch it! There's a rotten floorboard somewhere just here.
In British English, you can also use `Mind', followed by a noun referring to something the other person might hit, fall into, or harm, or a clause referring to something they must be careful about.
Mind the pond.
Mind you don't slip.
`Watch' is sometimes used in a similar way, especially with a clause.
Watch where you're putting your feet.
Other warning expressions are `Look out' and `Watch out'. `Look out' is used only in urgent situations of danger. `Watch out' is used for urgent situations and for situations that are going to arise or might arise, or, in American English, as `Mind...' is used in British English.
Look out. There's someone coming.
Watch out for that tree!
`I think I'll just go for a little walk.' – `Watch out – it's a very large city to take a little walk in.'
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