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.

to RADICAL doubt so that he
could build a bedrock belief
The Big Bang Theory Season 12, Episode 10

Here's a RADICAL thought.
The Big Bang Theory Season 5, Episode 10