sing 和 hum 这两个动词均含“唱”之意。
sing : 普通用词，泛指一般的歌唱或发出悦耳的声音。
- 晚会很豪华，食物精美可口，香槟酒源源不断。笑声和歌声交织在夜空中。
The evening was a lavish affair with glorious food and an endless supply of champagne. The sounds of laughter and singing mingled in the evening air.
hum : 指闭着双唇而不唱出字来，低语般的哼唱。
- 那个迎宾员在轻声哼着曲子。
The usher was humming softly to herself.
