【例1】 The children________(get) lost in the woods; otherwise, they would have been at the lakeside camp as scheduled.
【解析】 根据后文 they would have been 可知动作发生在过去，用“情态动词＋have done”。由句意可知“他们一定是在森林中迷路了”。
【答案】 must have got
【例2】 Harry is feeling uncomfortable. He ________(drink) too much at the party last night.
【解析】 must 用于对过去事情的推测。must have done 一定做过某事。句意：Harry 正感觉不舒服，他一定是在昨晚的派对上喝了很多酒。
【答案】 must have drunk
【例3】 Since nobody gave him any help, he________have done the research on his own.
【解析】 表示对过去肯定的猜测，故用 must have done，句意：没有人帮他，一定是他独自做的实验。
【答案】 must
