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, I SUPPOSE
I could get a haircut.
The Big Bang Theory Season 12, Episode 23

I SUPPOSE it does seem strange
from a foreigner's perspective.
Game of Thrones Season 3, Episode 9

I SUPPOSE that could be
The Big Bang Theory Season 12, Episode 24

Fine, fine. I SUPPOSE he can join us.
Game of Thrones Season 5, Episode 10

I SUPPOSE there's only one way to find out.
Game of Thrones Season 2, Episode 5