按照语法规则，非谓语动词作状语时，其逻辑主语应当与句子主语一致（这种规则叫做“依附规则”）。否则，即认为是错误的（即“悬垂错误”)。比较：
(1) Looking out through the window. we saw a beautiful landscape.
(2) (误) Looking out through the window, a bird flew away.
分析与总结：例（1）中状语分词的逻辑主语是句子的主语we，该句是正确的。例（2）中的状语分词的逻辑主语不是句子的主语a bird，该句是错误的，犯了悬垂错误。
然而，在语言的实际运用中，悬垂的非谓语动词状语在报刊、名家作品和科技文章中常可见到，因此，我们对悬垂分词或不定式的可接受性应具体问题具体分析，不可一概斥为错误。
多数现代语法家认为，在一定的上下文中，当意义不致于引起误解、歧义或语义混乱时，这种悬垂的分词或不定式是可以接受的。但前提是句子中必须有可以暗示出分词或不定式的逻辑主语的因素。本文拟结合具体实例探讨这类结构中暗示分词或不定式的主语的因素及暗示出的主谓。
(3) Looking out though the window, a beautiful landscape was seen.
(4) Standing on the church tower, the whole village could be seen.
(5)To avoid being attacked, strongholds were built.
分析与总结：上述三个例句均为被动语态，由此我们可以想象“往窗外看的”，“看整个村庄的”，“修堡垒的”是“某个（些）人”，这个“某人”便可理解为分词或不定式的逻辑主语。
(6) Getting down from the bus, my ankle was sprained.
(7) He felt himself gently touched on the shoulder; and looking around, his father stood before him.
(8) Leaving the cinema, it seemed to him that the film had been exceptionally bad.
(9) Searching along the deck, it had taken him some time to find a doctor.
(10) Walking or sleeping, the subject was always in my mind.
(11) Looking out through the window, a beautiful landscape presented itself before our eyes.
(12) Having been deserted by his guide, there seemed little hope that the explorer would find his way though the jungle.
分析与总结：上述诸例均通过人称代词的不同格形式暗示出状语分词的逻辑主语。例（6）中的my (ankle)暗示出分词主语是I；例（7）中的his( father)...before) him 及并列连词and连接的上下文暗示出分词的主语是he；例（8）中的(to) him 暗示出分词的主语是he；例（9）中的(it had taken) him……暗示出分词主语是he；例（10）中的(in) my (mind)暗示出分词的主语是I；例（11）中的(before) our (eyes)暗示出分间主语是we；例（12）中的(find) his (way)...暗示出分词主语是he (the explorer)。
(13) When dining in the restaurant, a jacket and tie are required. (When one dines...)
(14) When driving carelessly, it is easy to have an accident.
(15) When inquiring about a room it is always a good idea to ask at the beginning whether laundry and heating are included in the price.
(16) To borrow books from this library, it is necessary to register as a member of the library. (For one to borrow...)
(17) To master a language, words and grammatical rules are not enough.
(18) To be considered for college, an aptitude test must be taken.
(19) To become a champion swimmer, shoulder muscles must be developed while young.
(20) Being Christmas, the government office were closed. (Since it was...)
分析与总结：上述诸例均说明普遍存在的情况，即适用于每一个处于该情况的人，因此分词或不定式的逻辑主语是表示泛指的one，we或you。这种情况下悬垂的不定式较为常见。例（20）中分词的隐含逻辑主语是代词it，指时间，该句表示有规律的普遍情况。
(21) The siren sounded, indicating that the air raid was over. (... which indicated that... )
(22)Unknown lo his closest advisers, he had secretly negotiated with an enemy emissary. (It was unknown to his closest advisers that...)
分析与总结：上面两例中的分词说明的是整个主句的内容情况，故该分词的逻辑主语就是整个句子，我们可用which，as，it等（关系）代词表示。
(23) When treating patients with language retardation and deviation of language development, the therapy consists, in part, of discussion of the patient’s problems with parents and teachers, with subsequent language teaching carried out by them.
(24) Using the electric energy, it is necessary to change its form.
(25) When installing a boiler, the floor space which is available is very important.
(26) To check on the reliability of the first experiment, the experiment was replicated with a second set of subjects.
(27) To determine the number of cells, a sample is put under a microscope.
分析与总结：上述诸例均是科技语体文。正式的科学论著中，悬垂分词或不定式的逻辑主语就是作者或读者的I, we或you。科技语体文中通常避免使用人称主语，故悬垂分词或不定式状语分句在科技文章中是很常见的。
(28) Putting it mildly, you have caused us some inconvenience.
(29) His moral principles, to be frank, begin and end with his own interests.
分析与总结：上述两例中的悬垂分词和不定式状语是修饰整个句子的（即做评注性状语），用以表明说话者的态度、观点、看法等。因此该分词或不定式的逻辑主语就是讲话者本人，一般是I或we。这种用法的分词或不定式已成语化。常见的有strictly/generally/frankly speaking, considering that..., judging from.., speaking purely for..., to tell you the truth, to be sure/precise/fair, so to speak等。
但若从句子中不能判断出悬垂分词或不定式的逻辑主语时，这种结构便是错误的。如：
(30) (误) Reading the evening paper, a dog started barking.
分析与总结：上例中无法判断出分词的主语是谁，故是错误的。
全文将毕，笔者的体会是，尽管语法是描述语言规则的，但语言实践中的一些现象却是语法描述难以企及的。语法是人们总结出来的，它并不能包罗语言之万象。本文介绍的不合语法规则的语言现象初学者虽不宜模仿，但作为一种语言存在，却是值得研究和注意的。
（段惠彬 原载于《英语学习》1998年第5期）
本文由书生小二郎原创。
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