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。
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。
4． 否定结构的倒装语序
我们有时为了强调而把否定词和词组放在句首，这时句子结构应倒装。例如：
1） On no account should we follow blindly.我们决不应当盲从。
2） No where has the world ever seen such great enthusiasm for learning as in our country.没有任何其他地方有我国这样高的学习热情
