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.

Well, so am I.
Could I interest you in a nightcap?
The Big Bang Theory Season 9, Episode 24

...don't you think that
you COULD return the favor?
The Big Bang Theory Season 3, Episode 7

Could it be me?
The Big Bang Theory Season 1, Episode 12

Yeah, you know, we
COULD say a few words,
The Big Bang Theory Season 12, Episode 10

It COULD get you killed.
It COULD get me killed.
Game of Thrones Season 4, Episode 8