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.

but I spent most of FIFTH grade
with my head in a toilet.
The Big Bang Theory Season 7, Episode 2

...to be picked for a game in FIFTH grade,
I'm sure you do suck.
The Big Bang Theory Season 8, Episode 3

Before that, I was in college,
and before that I was in the FIFTH grade.
The Big Bang Theory Season 1, Episode 4

I think
it's only the FIFTH stage
The Big Bang Theory Season 12, Episode 24

You, uh, moving into the apartment
on the FIFTH floor?
The Big Bang Theory Season 6, Episode 8