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.

Thirty, FORTY, fifty coffees a day.
Money Heist Season 3, Episode 7

Forty thousand dollars?
Hey, you said raise the prices.
Breaking Bad Season 2, Episode 9

Forty horses died in the night.
Game of Thrones Season 5, Episode 7

Forty, Your Grace.
Game of Thrones Season 2, Episode 8

Forty milliliters.
The Big Bang Theory Season 4, Episode 7