Tom ________ better than to ask Dick for help.

A.shall know  B.shouldn't know  C.has known  D.should have known

【答】此题在命题上不够严谨，因为除了选择D 之外，还有两个可能的答案，就是选：A.shall know 和 C. has known。下面，分析这3句的区别：

1. Tom should have known better than to ask Dick for help.

2. Tom shall know better than to ask Dick for help.

3. Tom has known better than to ask Dick for help.

句1：汤姆本应该懂得事理不叫迪克帮忙。（实际上他不懂事，而是让迪克帮他了。）

句2：汤姆一定要懂得事理，不能叫迪克帮忙。（只是一种告诫或义务，并没有实施。）

【注】shall用于第三人称，表示“义务” ，或者表示说话者的意图、告诫、威胁、命令、决心、注定的结果等，意为“应该；必须；一定会；一定要”。例如：

He shall be sorry for it one day, I tell you.  有一天他会后悔的, 我告诉你。 (警告)

来自《用法词典》

As a man sows, so he shall reap.  种瓜得瓜, 种豆得豆。

来自《简明英汉词典》

He shall have a bicycle for his birthday.  他可以得到一架自行车作为生日礼物。

来自《现代英汉综合大词典》

He will die before he shall disgrace himself.  他宁肯死而决不受侮辱。

来自《现代英汉综合大词典》

I assure you, sir, that he shall be disciplined, severely disciplined!  先生,我保证, 他将受到纪律处分, 一定从严惩处!

来自辞典例句

If a man will begin with certainties, he shall end in doubts.  如果一个人以种种肯定的立论开始, 他必将以各种怀疑而终止。

来自辞典例句

He shall want for nothing that money can buy.  只要钱能买到的一切他都不会缺少。

来自辞典例句

句3：汤姆很懂得事理，没有叫迪克帮忙。（现在完成时表示“事件的结果，或对现在的影响”，说明汤姆确实很懂事，没有请迪克帮忙。）

【语言点】比较级结构，有时表达一种“取舍关系”: “more（或more 的替代词）... than ...”，than 之前的部分为“取”，than 之后的部分为“舍”。如果把than 之前的部分当做A，than 之后的部分当做B，那么，应这样翻译：“是A，不是B”，或者“与其说是B，不如说是A”。

know better than to do sth. 字面意思就是：人懂事的程度超过了做某事。引申为：人很懂事，而不做蠢事。这样than 的意思就含有否定之意了。

She is more like a student than a teacher. 她像一个学生，而不像老师。

They say it is more a comedy than a war movie. 听说这是一部喜剧片而不是战争影片。

Jack is more a writer than a poet. 与其说杰克是位诗人，倒不如说是位作家。

She is more a writer than a scholar. 与其说她是学者，不如说她是作家。

He is more like an artist than a carpenter. 他不像个木匠，更像个艺术家。

Tom is more foolish than stupid. 与其说汤姆笨，倒不如说他傻。

John is more a scholar than a teacher. 与其说约翰是位教师，倒不如说他是个学者。

He is more a sprinter than a swimmer. 他是短跑健将，而不是游泳选手。

It is more like a children's game than a real issue of life and death. 这与其说是真正的生死搏斗，不如说更象一场儿戏。

In some ways she is more like her father than her mother. 在某些方面,她更像她父亲而不像母亲。

What we have got here is more like a quarrel than a debate. 这里的样子不像在讨论而像是在吵架。

【补充解答】曹老师把这句话理解为：

Tom should have known better than to ask Dick for help. 对过去的情况进行推测：很可能汤姆头脑清楚，没有让迪克帮忙。

我认为这样理解有误。我前面已经翻译过了，这句话的含义是：

汤姆本应该懂得事理，不该叫迪克帮忙。（实际上他不懂事理，而让迪克帮忙了。）

这样的例句很多，又如：

You should have known better than to talk like that. 

你本应更懂事而不致与说这样的话。

You should have known better than to go mountain climbing alone. 

你本来不该笨得单独去爬山。

He should have known better than to take the risk. 

你应该很明智，不该去冒险。

▲ should 之后，可以接不定式的完成时表示推测，但这种用法是指“在某个时间之前应该业已完成的动作”，一般要跟 “by + 时间” 连用。例如：

The paint should have dried off by this time tomorrow. 油漆到明天这个时候就应该已经干了。

They should have arrived in Beijing by this time. 他们这个时候该到北京了。

I think I should have finished it by then.  我想到那时我应该完成了。

We should have the whole deal sewn up by the end of this week. 到本周末，我们应该可以把整个交易谈妥。

The letter should have arrived by now. 信函现在该到达了。