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 abandon his subjects to an evil that
he was too GULLIBLE to recognise?
Game of Thrones Season 4, Episode 3

Leonard, what did we say about
being a GULLIBLE weenie?
The Big Bang Theory Season 7, Episode 21

Au contraire, it meant she was GULLIBLE
and open to a little probing.
The Big Bang Theory Season 3, Episode 12