1. 独立结构：当分词作状语有自己的逻辑主语时叫独立结构，如：
　He being absent (= Because he was absent), we could do nothing.（原因）
　The boy ran into the room, his face and hands covered with mud(= and his face and
　hands were covered with mud).（伴随状况）
　His work finished(= After his work was finished), he went straight home.（时间）
　Weather permitting(= If weather permits), we'll go mountain-climbing tomorrow.（条件）

　2. 无归属结构：一般说来，分词或分词短语作状语，其逻辑主语应和句子主语一致，但在某些句子
　中（尤其是在科技英语中），分词的主语并不是句子的主语，而是包括在句子的某个成分中，这叫无归属结构，如：
　　Her summer holidays were spent in the countryside, helping the farmers with their
　　work. (helping 的逻辑主语不是句子的主语holidays，而是其定语her)