题目内容：

They ought to have finished their composition last Sunday ,____ they?
答案是didn't.但有书上又有填oughtn't的 这是怎回事?请回答得越详细越好.

优质解答

ought to 特殊,为情态动词翻译疑问句用oughtn't或shouldn't.在这里翻译为义务、责任应当,应该,也可用didn't.