He’ll definitely come back, so I’m okay.
You should always take out your contacts before going to bed. Otherwise your eyesight will get worse.
I will certainly complete WaniKani no matter what.
This word looks like a verb but it's not (sorry!). It's an adverb. The kanji itself means certain and the vocab version means all sorts of variations on that: surely, certainly, and without exception to name a few.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
Ra, the Egyptian god asks you: "Can you do it? Are you certain you can build the pyramids?"
"Yes I can ra (かなら). Yes I can," you reply.