Painful
Ow, It Hurts, Ouch
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of pain? It's painful. It's also when you say ow and ouch.
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: An eater (いた) is someone who eats. But, the eater hurt his mouth. So now it's painful to eat.
この部屋じゃ煙が目に入って痛いので、会社は屋外に喫煙場所を指定するべきだと思うね。
Our company should designate an outdoor smoking area because the smoke gets in my eyes in this room and it hurts.