sing-sings(单三)
例句：She sings very well.
sing-sang（过去式）
例句：We sang a song last night.
sing-is/are singing(现在进行时)
例句：we are singing now.
sing-was/were singing
例句：He was singing English songs when his sister came back.
sing-will sing
例句：I will sing songs tomorrow at the concert.
