Two tuning forks produce sound waves with the same frequency. If one tuning fork is struck harder, what aspect of the sound will change?

Striking a tuning fork harder gives it a larger amplitude of vibration. A larger amplitude means more energy in the wave, so the sound is louder. This is a change in intensity.

Pitch depends on frequency, and the frequency here stays the same. Quality (timbre) depends on the overtones present, which also stay the same.