必定；推测 (过去式为must + have + p.p.)
She's smiling at you. I think she must know you.
You have been working all day. You must be tired.
She looks pale. She must be sick.
He didn't come. He must have forgotten our date.
I lost one of my gloves. I must have dropped it somewhere.
He got full marks on the English test today. He must have studied hard last night. 他今天英文考试满分，昨天晚上他一定很用功。
When I woke up this morning, the light was on. I must have forgot to turn it off last night. 今天早上我醒来时，灯是亮的，我昨晚一定是忘了关了。内容来自 听力课堂网：http://www.tingclass.net/show-5855-216159-1.html