Correct sentence:
When the giant blows his nose, it sounds like a trumpet.
Why this is better (easy words):
- We say "blows his nose" when someone pushes air out to clear their nose. We don’t say "blows with its nose."
- "When" tells us one thing happens because of another (the blowing makes a sound).
- "It" means the sound the giant makes.
Other simple ways to say it:
- "When the giant blows his nose, the sound is like a trumpet."
- "The giant's nose sounds like a trumpet when he blows."
Nice sentence! You can choose the version you like best.