2. They think the stones can prevent illness and keep people healthy.他们认为这些石头可以预防疾病,使人保持健康。
prevent作及物动词，可是直接带宾语，表示“阻止，阻挠”的意思；常构成：protect...from...表示“保护……不受……侵袭”；prevent sb. from doing something. 阻止某人做某事，在主动语态中from可以省略，被动语态中不行。
3. “And perhaps we might never know , but we do know they must have been hardworking --- and great planners!”保罗?斯科特说“也许我们可能永远不会知道,但我们知道他们一定是勤奋——伟大的计划!” must have been doing: 表示肯定的猜测，肯定某人一直在做的事，并且猜测这种动作,或者行为一直持续到现在。
must have been done是must have done的被动形式，是对过去发生的动作最有把握的猜测，意思是“一
语法重点三 语法重点二 定”。
如：Since the road is wet,it must have rained last night.
They must have learnt 5000 English words by
the end of last term,hadn't they? The room must have been cleaned yesterday,
wasn't it?
语法重点一
1、 But I’m not sure if he likes rock music。 但我不知道他是否喜欢摇滚乐。
词组be sure, make sure 在祈使句中表示“ 确保、肯定”义。
如：Be sure you will get there at six in the morning. Make sure the answers are right.
sb is not sure 意为“某人不知道”， 相当于动宾结构的sb doesn’t know.
He isn’t sure when the class meeting will begin. = He doesn’t know when the class meeting will begin.
语法重点二 2. So I might be a bit late for the party.因此我可能聚会要晚到一点。
a bit和a little都作“一点儿”讲,但用法不同。 （ 1）a bit和a little在肯定句中修饰动词、形容词、副词和比较级时可以互换,表示\一点儿\的意思。 如: That's a bit too expensive. The speaker spoke up a bit/a little to make himself heard more clearly. （2）a bit和a little在否定句中的意思恰恰相反,not a bit相当于not at all“一点儿都不”;not a little相当于very(much)或extremely“很、非常”。 She is not a bit tired. =She is not tired at all. She is not a little tired. (3)a bit用作定语时，意为“一点儿”，不能直接修饰不可数名词，但可用“a bit of ＋不可数名词”结构，此时a bit of ＝ a little． He has a bit of／a little money left． She knows a bit of ／a little French
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