求定语从句中关系介词和关系副词要怎么确定!如题 看语法书上的看不懂 请教高手说明一下要怎么确定关系介词和关系副词?什么情况下一定得用用这个关系副词（关系介词） 为什么?

没有关系介词这一说法,只有关系代词和关系副词.如何确定,要根据他们在从句中所作的成分来确定,如果作主语或宾语,用关系代词,如：I know the girl who/that is standing in front of the building.who 在句中作主语.I know the girl who/whom/that our teacher is talking to.who/whom/that 作宾语.指物时,用which/that.事.如果作状语,用关系副词 when,where,why,如,I will never forget the place where I met the famous star.Please let me know the reason why you are late again.要注意的是why的先行词只有reason.