从句是一个完整的句子,则用关系副词.比如i don't like the restaurant where we have dinner with Lily.
“we have dinner with Lily”不缺少任何成分,是一个完整的句子,所以只需要加个where就好；
从句成分不完整,也就是缺主语或谓语或宾语,则用关系代词.比如 i don't the park which we visited last summer.“we visited last summer”不完整,visit缺宾语,所以需要加一个关系代词.