②-The woman biologist had studied wild animals in Africa for 13 years before she returned.

　　-Oh, dear! She_must_have gone through a lot of difficulties.

　　③-Is there a flight to Paris this evening?

　　-There might_be. I'll make a call and check it out.

　　④She_may not know about it.

　　⑤You are joking. You can't be serious.

语法点二 could, should和would的用法 　　1.could的用法

　　(1)could表示推测，表示某事有可能发生或可能是事实。在肯定句中没有must那么有把握；在否定句中没有can't那么有把握。

　　He couldn't be a bad man.

　　他不大可能是坏人。

　　(2)could还可以用于表示客气、委婉或礼貌的请求语气。

　　Excuse me, could you tell me the way to the bus station?

　　请问你能否告诉我去车站的路？

　　I'm afraid that we couldn't give you any definite answer at this moment.

　　恐怕我们这时还无法给您任何确切的答复。

　　(3)could have done表示"可能已经......"。此外，它还可以表示"本能够做某事(而没有做)"，暗含惋惜之情。

　　You could have done it well, but you were too careless.

　　你本可以做好的，但是你太马虎了。

　　Where could she have gone last night?

　　她昨晚可能到哪里去了呢？

　　2. should的用法

　　(1)should的基本用法。

　　①表示"应该，应当"。

　　②表示"可能，该(＝will probably)"。

　　③表示"万一"。

　　④表示"竟然"。

　　⑤表示粗暴地拒绝一项建议、要求或指示。

　　You should help your mother with the housework.

　　你应该帮你的母亲做家务。

He should arrive soon.