I’m mainly studying Japanese.
Primarily red and white cars are sold in Portland.
He is a researcher who is mainly interested in bananas.
主 means main so when you add に it becomes the adverb mainly.
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:
You mainly spend your time talking to your master Moe. But you mainly address him like, "Oh Moe The Great!" (おも). That's because he's above you. Imagine yourself saying "OH MOE THE GREAT!" as you walk into his room, bowing low.