三、 表达原因的结构
考生病句：
1. The real reason to our failure is not far to seek.
2. The reason for this is because some people want to earn plenty of money without working hard.
正确表达：
1. The real reason for our failure is not far to seek.
2. The reason for this is that some people want to earn plenty of money without working hard.
评议与分析：
以上两个病句分别引自92年1月和97年12月四级考试的考生作文。从遣词造句上看，这两位考生具有一定的写作能力，not far to seek, plenty of money以及without working hard等均运用正确、恰当。但令人遗憾的是，第一位考生不知道reason不与to搭配而应接介词for，第二位考生犯了一个中国学生常犯的错误，就是用because引起表语从句，because这个词不能引起表语从句，在本句中只能改用that才正确。
掌握好表达原因的结构是十分重要的，几乎所有的写作试题都要求写原因或可以写原因。在大学英语四、六级考试、研究生入学英语考试以及TOEFL考试中，写作的文体基本上是议论文，而议论文的基本模式是摆事实、讲道理，讲道理就是说明原因。写作测试的文体决定了表达原因结构的重要性。
英语中用来表达原因这一概念的结构有多种。我们可用as , because, since, seeing that, considering that, now that, not that...等词组引出表示原因的从句。例如：
1. Now that we have seen these great achievements with our own eyes, we feel more proud than ever of our country.
2. Professor Liu is strict with us because he wants us to make rapid progress.
3. Since we live near the sea, we enjoy a healthy climate.
4. Pollution is still a serious problem, not that we don't have the ability to solve it, but that some people have not realized the consequences of the problem.
我们还可以借助某些词语用简单句表达原因结构。例如：
1. The reason for this change is quite obvious.
2. Diligence is the key factor of success.
3. Idleness is the root of all evils.
4. He was ashamed to have made the mistake.(=He was ashamed that he had made the mistake. =He was ashamed because he had made the mistake.)
除了上述的例句外，英语中还有很多或易或难的表达原因的结构。我们在进行写作训练的时候，不能满足于一知半解，要讲究书面语言的正确性和准确性。比如，because是最常用的引导原因从句的连词，语气最强，表示直接的原因，若because置于句首，后面的主句不能再用so。用as引导的原因从句语气较弱，所说明的原因是附带的，而since表示的原因暗示着是稍加分析之后才能推断出来的原因。
四、 否定结构
考生病句
1. Some people think we needn't to worry about fresh water.
2. Nowadays many people don't like to go to the movies, too.
正确表达：
1. Some people think we needn't worry (或don't need to worry) about fresh water.
2. Nowadays many people don't like to go to the movies, either.
评议与分析：
例句1选自96年1月四级考生作文，例句2选自92年1月六级考生作文。例句1 的错误在于该考生混淆了need作为情态动词和作为普通动词的用法。
need作为情态动词时，主要用于否定句，后面的动词不带to, needn't worry，作we的谓语。need作为实意动词时，可用于肯定句、否定句和疑问句，don't need to worry 中的to worry作don't need的宾语。例句2的错误在于该考生混淆了too和either的区别，这两个词都表示"也"的意思，但是在英语中too, also只能用于肯定句，而either只能用于否定句。
否定结构除了在助动词、情态动词，be和have后面加not之外，还有许多不含not的否定结构。若能正确使用他们，文章会显得生动活泼，增加写作的闪光点。下面我们就来看看:
1． 含有否定意义的词汇和短语
以下列举的词和词组本身就具有否定的含义，因此无需用否定词。
介词against, beyond, but, except, without,...
形容词和动词absent, deny, differ, different, fail, free, ignore, miss, refuse, the last, used to, reluctant, lack, want,...
短语keep...from, protect...from, prevent...from, let alone, at a loss, in vain, instead of, out of the question, rather than, too...to, by no means, anything but,...
我们看以下例句：
1） Women fail to get the equal rights in some countries.
在一些国家里妇女没有得到平等的权利。
2） This is by no means the best way to solve the problem of energy crisis.
这不是解决能源危机的最好的办法。
3） We should protect trees from being destroyed.
我们应保护树木，不让它们受破坏。
4） In old China we could not make a nail, let alone(make) machines.
在旧中国，我们连一个钉子都造不了，更不用说制造机器了。
2． 含有半否定意义的词语
barely, hardly, few, little, rarely, scarcely, seldom, not all, not everyone, not everything,...具有半否定的意义。例句：
1） We could hardly see any fresh vegetables in winter on market several years ago.
几年前在冬天市场上很难见到新鲜蔬菜。
2） These young people know little about how to choose good books to read.
这些年轻人几乎不知道如何挑选优秀的书籍来读。
3． 不含否定意义的否定结构
有些词和词组形式上是否定结构，但其含义是肯定的，比如：cannot but, can't help, no sooner...than, not...until, in no time, none other than, nothing but,等等。例句：
1） We can't but face the reality.
我们只有面对现实。
2） These old buildings will be replaced by modern apartment buildings in no time.
这些旧建筑将很快为现代化的公寓所代替。