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.

- Careful, Imp.
- I've offended you.
Game of Thrones Season 1, Episode 4

Careful, Leonard. Liquid nitrogen,
320 degrees below zero.
The Big Bang Theory Season 1, Episode 5

Be CAREFUL.
Money Heist Season 1, Episode 4

- You betrayed her.
- Careful now.
Game of Thrones Season 5, Episode 10

Be CAREFUL you don't out yourself.
Game of Thrones Season 1, Episode 2