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, rest ASSURED,
I am in full possession of my faculties.
The Big Bang Theory Season 4, Episode 3

Maester Wolkan
has ASSURED us beyond all doubt.
Game of Thrones Season 5, Episode 5

Dr. Cooper ASSURED me
this is what you wanted.
The Big Bang Theory Season 12, Episode 5

But rest ASSURED that...
Breaking Bad Season 3, Episode 8

Maester Pycelle ASSURED me
your wounds were not fatal.
Game of Thrones Season 3, Episode 1