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.

I heard you had
some kind of a MENTAL break?
Westworld Season 3, Episode 8

I forget, which MENTAL hospital
are you guys registered at?
The Big Bang Theory Season 11, Episode 10

And with that MENTAL picture,
I think we're done for the evening.
The Big Bang Theory Season 4, Episode 4

You need to push your brain out of its
comfort zone and reach MENTAL exhaustion.
The Big Bang Theory Season 10, Episode 22

who don't have MENTAL problems.
The Big Bang Theory Season 9, Episode 10