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.

Let me ASSURE you, we will not
engage in workplace coitus.
The Big Bang Theory Season 12, Episode 5

I ASSURE you a madman
is scarier than a skeleton.
Money Heist Season 1, Episode 1

She's flowered, I ASSURE
you. She and I have discussed it at length.
Game of Thrones Season 3, Episode 5

I ASSURE you that cutting
a dumpling in thirds is child's play...
The Big Bang Theory Season 1, Episode 7

Is this about the temperature setting?
I can ASSURE you that will not happen...
Breaking Bad Season 3, Episode 8