我说下非限制性定语从句吧。定语的作用就有划范围和补充的作用。e.g.I like the girl who is with long hair.
Oil rigs have to be repaired frequently and divers,who often have to work in darkness a hundred feet under water,have been frightened out of their wits by giant fish bumping into them as they work.
说一个从句是非限制性定语从句,(逗号分离)就是可有可无。它仅仅是补充的一句话。当插入语理解。。回忆笔记的,大概区别就是补充和限定了。重心语还在后面。前几楼说到了。