举个例子：Standing on the tower,we could see the whole town below us.
这个句子中,分词（standing on the tower）作状语,其逻辑主语是we(we stand on the tower),
而句中（we could see the whole town below us中）的主语也是we,是一致的,故这个句子是正确的.而若句子写成,Standing on the tower,the whole town could be seen by us.那分词的逻辑主语和句子的主语（the whole town ）就不一致了,这个句子就错误了.
不过英语中有些分词短语已成为固定的惯用语,如generally speaking,不受这规则限制.如我们可以说Generally speaking,the book is good.虽然speak不是book发出的.