The best way to learn proper English is to read news report, and watch news on TV. Watching TV shows is a great way to learn casual English, slang words, understand culture reference and humor. If you have already watched these shows then you may recall the words used in the following dialogs.

This is an authentic Chinese throwing star,
and I must WARN you...
The Big Bang Theory Season 6, Episode 9

I should WARN you I have
a tongue piercing.
The Big Bang Theory Season 12, Episode 3

...someone called to WARN him.
Breaking Bad Season 3, Episode 9

All right, just WARN me before you do it.
The Big Bang Theory Season 10, Episode 8

But I WARN you,
The Big Bang Theory Season 12, Episode 12