Hard
Tough, Firm
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 hard? It's hard, firm, or tough.
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 need something really hard to put in your catapult(かた). Look around you for something hard and imagine putting it in your catapult!
このお肉、ちょっと固い。
This meat is a little tough.
父の決心は固いと思います。
I think my father's decision is firm.
兄は、口が固いことで有名です。
My elder brother is known as someone who can keep a secret.