Question

【题目】He's doing his homework, but his sister(开始唱歌了).

Answer 1

【答案】starts to sing
【解析】开始做某事：start to do sth。开始唱歌：start to sing。his sister他的姐姐，是第三人称，动词用单三形式。start单三形式starts。故填starts to sing。