hum a tune的用法和样例：
- He liked being there so much that he began to hum a tune.
- She was humming a tune to herself.
- John was humming a tune when he put on his clothes.
- He was humming a tune as he walked along.
- He was humming a tune to himself.
- Little hedgehog alacrity thinking about, hum a tune out the door, leaving the aunt sheep.