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.

All right, imagine these two guys
had Patrick working as a MULE.
Breaking Bad Season 3, Episode 12

were to sell his MULE
and his last pair of shoes,
Game of Thrones Season 4, Episode 2

What I don't see is the fucking MULE.
Where did you leave it?
Money Heist Season 3, Episode 5

until that MULE kicked him in the chest
and killed him.
Game of Thrones Season 4, Episode 8

as a MULE.
Money Heist Season 3, Episode 8