同位语从句中的连词that只起连接作用,在从句中不担当任何句子成分;而定语从句中的关系代词that在句中做宾语或主语,宾语与先行词有修饰关系。
   如果句子是同位语从句,就应用连词that而不能用which.同位语从句一般放在表具体含义的名词后解释名词的含义或内容,如以下名词:news、fact、suggestion、truth、plan、belief、doubt、possibility、idea等,而定语从句只是对先行词的限定和修饰。
如:
  They expressed the hope that they would come to visit China again.(同位语从句)
  The hope she expressed is that they would come to visit China again.(定语从句)