1) must用在肯定句中表示較有把握的推測，意為"一定"。
2) must表對現在的狀態或現在正發生的事情的推測時， must 後面通常接系動詞be 的原形或行為動詞的進行式。
you have worked hard all day.you must be tired. 你辛苦幹一整天，一定累了。（對現在情況的推測判斷）
he must be working in his office. 他一定在辦公室工作呢。
比較：
he must be staying there.
他現在肯定呆在那裡。
he must stay there.
他必須呆在那。
3) must 表示對已發生的事情的推測時，must 要接完成式。
i didn't hear the phone. i must have been asleep. 我剛才沒有聽到電話，我想必是睡著了。
4) must表示對過去某時正發生的事情的推測，must 後面要接不定式的完成進行式。
---why didn't you answer my phone call?
---well, i must have been sleeping, so i didn't hear it.
5) 否定推測用can't。
if tom didn't leave here until five o'clock, he can't be home yet. 如果湯姆五點才離開這兒，他此時一定還未到家。