且都没有逻辑错误和语法错误，那就产生歧义了。
Example：
Ambiguous: I like cheese more than Yvette. {Yvette could be subject or object.} Yvette在原句中既可以做主语，表示“我比Yvette更爱芝士”；也可以做宾语，表示“我爱芝士比我爱Yvette多”，产生了歧义。
? 如何消除歧义
1) 当比较对象做宾语时，将省略的主语和谓语都补充完整
Right: I like cheese more than I DO Yvette. (= than I like Yvette) 如何补充完整：把than前面的比较对象cheese换成Yvette，谓语like由助动词代替，然后照抄到than后面。
2) 当比较对象做主语时，将助动词补充进来
Right: I like cheese more than Yvette DOES. (=than Yvette likes cheese) Yvette做主语，助动词要变成第三人称单数，cheese是相同的宾语，可以省略。
平常s1 v o at the same time as s2 有s2与o比较的歧义，所以为了避免造成这种歧义，就把动词补助，将其限定为s1与s2两主语的比较，如果这个动词后没有带宾语，不补助动词也不会引起歧义，补了助动词反而是redundant
? 如果句子本身不存在歧义，多加了助动词或本该省略的词，就比较
wordy了。但是，GMAT中也出现过这种wordy的现象，所以GMAC为王，千万不要以多余助动词的出现作为排除选项的标准，只是分析题目的时候，这个知识点要识别的出来。
Example：
Right: Vishal eats more carrots than donuts. (donuts must be the object) Wordy: Vishal eats more carrots than HE DOES donuts.
? 隐喻式的比较是不被GMAT接受的，要用than使比较明确起来，所
以如果句子中没有than，就不可以用比较级！
Example：
Wrong: With winter coming, I will have HIGHER energy bills. Wrong: I will have HIGHER bills OVER last year. Right: I will have HIGHER bills THAN last year.
Always use than with a comparative form.
12、 动词和比较（高阶）
? 助动词
1) 在动作重复的时候，后半部分的动词和宾语可省略，用助动词be,
do, have/has代替。不过，如果前后部分动作一样但时态不一致时，便不能省略主要动词以及宾语，其中宾语可以由代词代替，避免重复。如下例： Example:
Wordy: I have never seen an aardvark, but my father has seen an aardvark. Better: I have never seen an aardvark, but my father HAS. Wrong: I have never seen an aardvark, but last year my father DID. Right: I have never seen an aardvark, but last year my father saw one.
2) 在一些罕见的情况下，and的前后动作时态可以不一样，如下例： Example:
Wrong: Our cars were designed to inspire envy, and they ARE. Right: Our cars were designed to inspire envy, and they DO.
分析：第二句中，“do”和前面部分的时态不一致，前面是一般过去时，后
面是一般现在时。由于they指代的是cars，根据逻辑句意知道，do后面省略的是inspire，而inspire和cars是主动关系，所以用do不用are。
3) do, be, have这些助动词永远代表肯定的意思，如下例：
Right: Some people do not eat soup. but others do. (= do eat soup) Right: Some people do not eat soup as others do. (= do eat soup)
Use be, do and have in this way only if you mean the positive form of the verb.
? 情态动词
1) 情态动词的位置：放在谓语动词前面 Example:
Wrong: Our division spent significant funds on HAVING TO build facilities. Right: Our division HAD TO spend significant funds on building facilities.
2) 如果句子中其他词语已经表达了情态动词要表达的意思，就不能再
出现情态动词以免重复。如下例： Example:
Wrong: This plan ensures that action MUST be taken. Right: This plan ensures that action WILL be taken. 3) “be to”在GMAT中不能用，得用will或should
“be to”本来表示“某种义务或者将来会做…”，在GMAT里不用“be to”表示这两种意思，转而用“will”和“should”表示将来或义务。(PS: should 不能表命令) 如下列： Example:
Wrong: We ARE TO receive an invitation.
Right: We WILL receive an invitation. OR We SHOULD receive an
invitation.
? 倍数的说法见曼哈顿P250 ? 其他比较结构
1) less或more在修饰“adj.+noun”时容易引发的歧义
wrong：We have even MORE efficient engines than before. Right: We have even MORE engines that are efficient than before. Right: We have engines even MORE efficient than before.
分析：错句里，more既可以修饰efficient，也可以修饰engines，引发歧义。
2) 没有逗号在中间的exceed和surpass也有比较的意思，要平行：
Right: The incidence of the disease among men EXCEEDS the incidence
among women.
Right: The incidence of the disease among men EXCEEDS its incidence
among women.
Right: The incidence of the disease among men EXCEEDS that among
women.
3) In addition+名词，表示“除…之外”
Rights: IN ADDITION TO taxes, death is inevitable.（主语比较） Right: IN ADDITION TO Munster cheese，I like Swiss.（宾语比较）
13、 代词
代词指代错误在GMAT中经常发生，所以每一次在题目中看到代词，都要去检查，看其是否指代正确！步骤是：
1） 找到代词的先行词（先行词一定是名词），并且看先行词是否唯一
2） 把先行词带入代词所在的句子或短语中，看逻辑是否错误 3） 看先行词与代词的单复数形式是否一致
PS：这个步骤不仅对于发现代词问题很重要，并且对主谓的检验也非常重要。碰见主谓时，都要问问自己，这个动词的主语是什么;主语和动词是否make sense together; 是否做到了主谓一致
错误一：先行词必须存在
关于这一点，GMAC经常设的圈套是，把本来是名词的词在剧中当形容词用，但是记住一点：先行词一定存在于句子中并且是一个名词！所以，在句中充当形容词角色的所谓“名词”，是不能充当先行词的，也就是说，该句子缺少主语！如下面这个例子：
Example:
Wrong：The park rangers discussed measures to prevent severe wildfires, which
would be devastating to IT.
Right：The rangers discussed measures to prevent severe wildfires, which would be
devastating to THE PARK.
在这个例子中，代词it的先行词应该是park，但是park在这里不是做名词，而是修饰rangers的形容词，所以这道题缺少先行词。
注意：Watch out for nouns used as adjectives! They cannot be antecedents of pronouns!!
The antecedent to which you want to refer must actually exist in the sentence and be functioning as a noun!!!!
错误二：先行词带入代词的句子或短语后，逻辑不对，没有make sense together!
错误三：一个代词有多个先行词的可能，导致ambiguous
Example:
Wrong: Researchers claim to have developed new \
cellulose fibers, which THEY allege give THEM the strength of cast iron.
Right: Researchers claim to have developed new \
cellulose fibers, which allegedly give THESE MATERIALS the strength of cast iron.
Wrong的例子中，“they”和“them”都既可以指代”researchers”也可以指代new ”nano-paper”,虽然通过上下文和逻辑可以推测，但不能百分百确定就是ambiguous！！！！
错误四：先行词和代词单复数不统一
这个和主谓一致类似，GMAC挖的陷阱也和主谓一致类似，详情请参考主谓一致。
特别注意：
1. 代词在句子主语的位置上，通常会被认为指代与其平行句子的主语。
EG: Supernovas destroy their immediate environments in vast explosions, BUT by
synthesizing heavy chemical elements, THEY provide the universe with the possibility of biochemistry-based life as we know it.
“They”在第二个句子中充当主语，虽然除了“supernovas”还有“environments”和“explosions”可以充当先行词，但基于以上所提的原则，“they”只能指代第一个并列句子的主语。注意：这条规律的前提条件是“平行的句子”，所以第一要平行，第二要是两个分开的句子，而不是一个句子中的主从句。 2. 主格和宾格代词不能指代所有格名词
Example:
Wrong: The board is investigating several executives' compensation packages in
order to determine how much may have been improperly awarded to THEM.
Better: The board is investigating the compensation packages of several executive in
order to determine how much They may have been improperly awarded.
Best: The board is investigating the compensation packages of several executives
in order to determine how much The executives may have been improperly awarded.
分析：在第一个句子里，宾格代词“them”不能指代所有格名词；在第二个句子里，虽然executives不再是所有格，可以指代，但是“them”的先行词既可以是executives也可以是packages，指代模糊；第三个句子为了防止以上两个句子的错误，把代词换成了那个本要被指代的名词，虽然重复，但是句子正确。千万要记住，concision（简洁）是选择答案最后才考虑的，不是首要考虑的！！！另外，主要到几个句子的所有格既可以用’s.也可以用 of.
总结：即代词不能指代 N’s X中的N, 但可以指代 X of N中的N。
3. which，that 只能指代物，只有who, whom 才能指代人
4. that 和those 指代的是一个new copy of 先行词，而不是完全相同的指代
Example:
The money spent by her parents is less than that spent by her children. The executives of her company are more excellent than those of competitors.
在第一个例句中，that的出现，是为了防止重复使用“the money“, 而不是指代“the money spent by her parents”. 这一点，it，they等代词却不同，他们都是指代完全相同的一个事物或人物。
在第二个例句中，those指代的是复数形式的先行词的new copies. 表示new copy的时候，Those和That也只能指代物，不能指代人指代new copy，单数用that，复数用those。 再如下例：
Lacking information about energy use, people tend to overestimate the amount of energy used by equipment. such as lights, that are visible and must be turned on and off and underestimate that used by unobtrusive equipment, such as water heaters.
(A) equipment, such as lights, that are visible and must be turned on and off and
underestimate that
(B) equipment, such as lights, that are visible and must be turned on and off and
underestimate it when