To sing — to produce musical vocal sounds; the common meaning of singing a song.
週末に友達とカラオケで歌った。
I sang at a karaoke with friends on the weekend.
鳥が朝早くきれいに歌っている。
Birds are singing beautifully early in the morning.
子どもたちが運動会で校歌を歌った。
The children sang the school song at the sports day.
To sing of or celebrate something in poetry or literature — to express in poetic form (e.g., to sing of love, beauty, or heroic deeds).
詩人は自然の美しさを歌った。
The poet sang of the beauty of nature.
古い民謡は村の生活を歌っている。
Old folk songs celebrate village life.