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.

Hi, sorry I'm late.
I was at the GYM.
The Big Bang Theory Season 4, Episode 5

That was a "Hey, Leonard, do you
want to go to the GYM?" yeah.
The Big Bang Theory Season 11, Episode 17

How dare you ambush my
girlfriend at the GYM!
The Big Bang Theory Season 5, Episode 4

There's a pool, a GYM--
The Big Bang Theory Season 7, Episode 1

and Penny's apparently
in the GYM all the time,
The Big Bang Theory Season 12, Episode 12