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.

Sir, if you have a COMPLAINT,
Breaking Bad Season 2, Episode 11

making a fake COMPLAINT.
Money Heist Season 1, Episode 4

Be ruthless. Tell him if he doesn't start
talking, we'll register a COMPLAINT...
The Big Bang Theory Season 4, Episode 19

Is this about a COMPLAINT?
I'd be happy to refer you
to our website.
Breaking Bad Season 2, Episode 11

an official COMPLAINT,
but if they do have a permit,
The Big Bang Theory Season 12, Episode 14