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.

Bend the knee,
I promise you MERCY.
Game of Thrones Season 5, Episode 1

Sometimes it is better to
answer injustice with MERCY.
Game of Thrones Season 4, Episode 4

Ah, congratulations,
and may God have MERCY on your soul.
The Big Bang Theory Season 6, Episode 3

Mercy.
Game of Thrones Season 4, Episode 7

The Mother have MERCY, then.
Game of Thrones Season 5, Episode 10