must
(1)必须，应该
the auditor must conduct sufficient verification. 审计师必须进行充分的验证。
(2) must not意为“不应该”，“不许”;need not是must的否定回答
we must not be afraid of difficulties. 我们不应该害怕困难。
must i return the book.? —yes， you must. 我必须还这本书吗?是的。
—no， you needn't. 不，不必。<>
(3) must + have + pp 表对过去的推测
i can't find my umbrella. someone must have taken it. 我找不到我的伞，一定是被谁拿走了。
(4) must + be + -ing 一定在
they must be talking about us now. 他们准在谈论我们。<>
(5)have to是must的同义结构，可表达现在，过去和将来时态。
we will have to change our plan. 我们得改变计划。