The challenge seemed impossible, but he refused to give up.
Meaning: The problem seemed impossible, but he kept trying to solve it.
Choice of Words:Impossible means unable to be done, and refused to give up means to continue trying even when it's difficult.
Alternative Expressions
The challenge was like a big mountain, but he kept climbing.
Meaning: This means that even though the task was very difficult, he didn't give up and kept trying to overcome it.
Choice of Words:Challenge is like a difficult task or problem. Mountain is something very tall and hard to climb. Kept climbing means to continue going up the mountain.
The challenge was like a puzzle, but he kept working at it until he solved it.
Meaning: This means that even though the task was very difficult, he didn't give up and kept trying until he found the answer.
Choice of Words:Challenge is like a difficult task or problem. Puzzle is something that you have to figure out by putting pieces together. Kept working means to continue trying to solve the puzzle. Solved means to find the answer to the puzzle.