怎样分辨动词在句首时是祈使句还是动词做主语?

动词及其非谓语动词形式一直是各类考试中的热点,更是我们在日常教学中的难点, 尤其是置于句首时该用何种形式更让学生头疼.其实只要我们掌握了它们的使用规律,要想做好这类题也不是难事.本文要讨论一下动词及其非谓语动词置于句首时该如何判别其正确形式.
一．动词原形
动词原形置于句首,毫无疑问应该是祈使句.祈使句的考查形式多是“句子+连词+句子”, 最常见的连词有and, or, otherwise, even though 等.所以判别的时候先看一下后边是否有一个“连词+句子”的形式；如果有,则句首的空格一定要填动词原形.连词前可以有逗号,也可以没有.
例1．
--English has a large vocabulary, hasn’t it?
--Yes. __ more words and expressions and you’ll find it easier to read and communicate.(04上海高考-44)
A. KnowB. Knowing C. To knowD. Known
答案选A. 因为后边有一个连词and, 其后是一个完整的句子,前半部分中没有主语,则一定是祈使句.
一．动词不定式
动词不定式置于句首,有两种功能,一种是做主语,另一种是做目的状语.
(一) 动词不定式做主语, 通常有两种情形: 一种是不定式直接放在句首(这种形式较少),另一种是用it做形式主语,把不定式置于句末,达到平衡句子的目的.
eg. To see is to believe.
It’s nice of you to help me with my English.
(二) 不定式置于句首做目的状语是考查的重点, 其后通常有逗号与主句隔开,相当于in order to.
例2 ____ this cake, you’ll need 2 eggs, 175g sugar and 175g flour.
A. Having made B. Make C. To make D. Making(06 广东高考-30)
答案选C. To make 表目的,相当于In order to make.
例3 –Can the project be finished as planned?
--Sure, __ it completed in time, we’ll work two more hours a day.(05 福建高考-29)
A. having got B. to getC. getting D. get
答案选B. to get表目的, 相当于In order to get.
三. V-ing 形式置于句首
V-ing 既可以是动名词,也可以是现在分词.
(一) 动名词置于句首,做主语,常表示概括性的或一般性的行为.
例4 ___ to sunlight for too much time will do harm to one’s skin.(02 上海高考-38)
A. Exposed B. Having exposed C. Being exposed D. After being exposed
答案选C. Being exposed to sunlight for too much time 在句中做主语.
需要注意的是, 动名词做主语和表语应遵循对称原则,即: 主语用动名词时, 表语也用动名词. 例如, Seeing is believing.
(二) 现在分词置于句首,做状语,表明分词的逻辑主语与主句的逻辑主语一致,其否定形式是not doing. 如果分词所表达的动作明显先于主句谓语动词的动作,则用其完成形式having done, 否定形式是not having done.
例5, My cousin came to see me from the country, ___ me a full basket of fresh fruits.(06安徽高考-21)
A. broughtB. bringing C. to bringD. had brought
答案选B. bringing在句中作伴随情况状语,其逻辑主语就是句子的主语my cousin.
现在分词视情况,有时其前面可以加上when, while, after, before, if, though, whether, unless 等连词, 这时相当于 “连词+主语(主句主语或it)+ be +v-ing” 形式分句作状语时的省略.
例6
When __ different cultures, we often pay attention only to the differences without noticing the many similarities.(o6浙江高考-5)
A. comparedB. being comparedC. comparingD. having compared
答案选C. 分词短语相当于状语从句 when we are comparing different cultures.
有些经常用于句首的-ing 形式短语已经转换成介词, 主要有 according to 根据, judging from 从……判断, talking of 谈到, taking all things into consideration 全盘考虑, setting aside 除开等. 另外还有一种常见的 “ 副词+speaking”结构置于句首做整个句子的状语.
Judging from her accent, she must be an American.
Taking all things into consideration, I think he is a very honest man.
Frankly speaking, his article still needs more revising.
四. 过去分词置于句首
过去分词置于句首做状语, 通常表示被动或完成, 说明谓语发生的背景或情况.
例7. ___ automatically, the e-mail will be received by all the club members.(06 上海高考-40)
A. Mailed outB. Mailing out C. To be mailed outD. Having mailed out
答案选A. 分词短语作状语, 相当于一个假设条件状语从句If it is mailed automatically, e-mail 与mail out 是被动关系.
例8. __ for the breakdown of the school computer network, Alice was in low spirits.(06 福建高考-33)
A. Blaming B. BlamedC. To blameD. To be blamed
答案选B. 非谓语动词形式作原因状语,主语是blame所表达动作的承受者, 因而用过去分词blamed.
过去分词置于句首还有一种比较特殊的结构, 即 “be+ 过去分词+ 介词” 结构表示状态的短语变化而来, 也就是 “be+ 过去分词+ 介词” 短语置于句首表示状态时, 可以直接省略掉be动词.
例9. __ with so much trouble, we failed to complete the task on time. (06 四川高考-33)
A. FacedB. FaceC. FacingD. To face
答案选A. 可以将该分词结构看作是be faced with 短语置于句首作原因状语的省略.
同现在分词作状语时前面可以加when, while, though, if 等连词的情况一样,过去分词前也常加一些连词.
例10. When first __ to the market, these products enjoyed great success. (04 四川高考-22)
A. introducing B. introduced C. introduce D. being introduced
答案选B.非谓语动词短语相当于when these products were first introduced to the market作状语的省略.
五. 独立主格结构
英语中最常见的独立主格结构就是 “名词(代词) + 现在（或过去）分词,位于句首做状语．在这种结构中,名词或代词是分词形式的逻辑主语,在意义上可以表示时间,条件,原因,伴随状况等,现在分词表主动,过去分词表被动．
Weather permitting, we will go picnic tomorrow.
The question settled, the meeting was over.
动词及其非谓语动词形式的用法看似复杂,实际上我们只要分析好句子成分及该动词在句中的角色,就比较容易把握好适当的形式了．