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.

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

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

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

Rest ASSURED, Your Grace, you can
count on the Iron Bank's support.
Game of Thrones Season 7, Episode 4