你知道什么叫悬垂分词吗？请看看这道题：
__________ the road，a car knocked him down.
A. Crossing B. Crossed C. When he was crossing D. To cross
此题应选C。其余均可能被误选。分析如下：
选项B不对，因为它是过去分词，其后不应有宾语。
选项A，D不对，因为该非谓语动词的逻辑主语与句子主语不一致——构成悬垂分词。
一般说来，作状语用的非谓语动词的逻辑主语应与句子主语保持一致，否则便是错句。比较：
为了通过考试，(他)每分钟都用来学习。
误：To pass the exam，every minute was spent in studying.
正：To pass the exam，he spent every minute in studying.
信读了第二次之后，意思就更清楚了。
误：Reading the letter a second time，the meaning became clearer.
正：Being read a second time，the letter became clearer in meaning.
狗叫得厉害，所以我把它放了出去。
误：Barking madly，I led the dog out.
正：Barking madly，the dog was let out.
但是，在有些特殊的结构(只有少数固定结构)中，非谓语动词可以没有逻辑主语：
Judging from [by] what he said，he was a cheat. 从他说的话来看，他就是个骗子。
