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.

How come you're not
doing a MISSION?
The Big Bang Theory Season 5, Episode 12

Yes. I accept this five-week MISSION to
share a living space with my girlfriend.
The Big Bang Theory Season 10, Episode 5

Ride south again with your MISSION complete
and your army intact.
Game of Thrones Season 6, Episode 8

This is a high-risk MISSION.
Money Heist Season 3, Episode 6

I can't go off on a secret MISSION
Game of Thrones Season 6, Episode 8