The patient has turned the corner after a difficult surgery.
I think it would be difficult to leap across this ditch.
"How do I do this?" "It’s not as hard as rocket science. Give me that. Let me show you."
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 difficult? It's also difficult.
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: It's awfully difficult to load a cow into your Moo-zooka (むずか). Click. Boom!