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.

Why would I be INTERESTED?
I have Penny.
The Big Bang Theory Season 6, Episode 2

you guys INTERESTED in a free screening
of an upcoming movie?
The Big Bang Theory Season 9, Episode 19

So are you INTERESTED in a little party?
The Big Bang Theory Season 2, Episode 21

Howard, relax.
I am not INTERESTED in your girlfriend.
The Big Bang Theory Season 3, Episode 10

Well, she might be INTERESTED to know
that I've refocused my research...
The Big Bang Theory Season 1, Episode 15