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.

Save him and I will free you. I SWEAR it.
Game of Thrones Season 1, Episode 9

I'd SWEAR you my sword,
Game of Thrones Season 6, Episode 10

no matter what you SWEAR.
Game of Thrones Season 8, Episode 4

Do you understand
and SWEAR to this?
Vikings Season 1, Episode 1

perhaps you should start
a SWEAR jar.
The Big Bang Theory Season 7, Episode 10