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.

Otherwise it's gonna get
infected and FESTER.
Game of Thrones Season 4, Episode 7

his wound must be washed
and sewn or it will FESTER.
Game of Thrones Season 1, Episode 8

you don't like "Fester"-- you're
just impossible to please.
The Big Bang Theory Season 7, Episode 15