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.

What better GUARANTEE
could you have
Game of Thrones Season 7, Episode 3

I can pretty much GUARANTEE it.
I wish I had your certainty,
Breaking Bad Season 5, Episode 15

There's no GUARANTEE our sperm's
going to generate high-IQ offspring.
The Big Bang Theory Season 1, Episode 1

And the very fact that you needed
a written GUARANTEE of respect
The Big Bang Theory Season 9, Episode 18

...and I can personally GUARANTEE...
The Big Bang Theory Season 6, Episode 9