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.

Oh, for fuck's SAKE.
Game of Thrones Season 5, Episode 6

For God's SAKE, Ma, I'm 26 years old.
The Big Bang Theory Season 1, Episode 11

Fly for God's SAKE.
The Big Bang Theory Season 2, Episode 19

Not just for their own SAKE,
but for everyone's.
Game of Thrones Season 6, Episode 4

Oh, for God's SAKE.
The Big Bang Theory Season 2, Episode 1