1. Something terrible must have happened if Li Chang was not coming to eat in his restaurant as he always did. (P10)
must have done 表示对发生在过去的事情的肯定的推测。如：
The classroom is empty. I think they must have gone to the library. 教室空了，我想他们一定去图书馆了。
The ground is wet. It must have rained last night. 地面湿了，昨天夜里一定是下雨了。
【拓展】 must do / be 是对现在或将来的事情表示推测；而must be doing则是对正在进行的事情表示推测。如:
Don’t call him. He must be sleeping at the moment. 别给他打电话了，此刻他一定正在睡觉。
Though I’ve never seen you before, I guess you must be the new secretary. 我以前没见过你，但我猜你一定是新来的秘书。
