How to use warn? (Tell me about grammar and prepositions)

It can only be warn sb not to do. If you want to use never, add it in the clause

warn verb

1. to tell someone of a possible future problem or danger so they can try no prevent it or avoid it warning, reminder

Usage: to warn of sth, to warn sb of sth, to warn that..., to warn sb that ...

Example: 1. The morning paper warned of serious delays at the airport.

2.He warned his students that they would have to work very hard to pass.

3.I warn you it's going to be very cold. I warn you that the weather is going to be very cold.

4.His boss had warned her about difficult customers.

His boss had warned her that some customers were difficult to deal with.

2.to advise sb not to do something because of a possible problem or danger in the future warning (someone not to do something)

Example: I warned her not to go near the dog, but she ignored me.

I warned her not to go near the dog, but she didn't listen.