当定语从句中缺少主表宾定语时,用关系代词,缺少状语时用关系副词.
1.This is the reason ___ he gave me for his being late.
2.The reason ___ he was late this morning is still unknwn.
例句1中 ___ he gave me for his being late是定语从句,从句中谓语gave缺少直接宾语,并且此处的直接宾语应该是前面的先行词reason,所以用关系代词.
例句2中 ___ he was late this morning 是定语从句he was late this morning 且主系表完整,缺少状语,所以选用关系副词.