同位语从句是作补充说明，从句句子一般不缺成分。定语从句是修饰先行词，从句一般缺少成分（主语，宾语，状语）。例如：同位语从句：Word came that the missing boy was found.that从句是补充说明Word（消息）。定语从句：The book that was written by Mo Yan is well-received.先行词是The book在从句中作主语。
