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.

This is a CLASSROOM.
This is not American Bandstand.
The Big Bang Theory Season 8, Episode 2

Every science CLASSROOM,
The Big Bang Theory Season 7, Episode 6