This is the room where we lived last year.这里为什么不能用关系代词that而用

where 是关系副词,表示地点,在句中作状语.而that在定语从句中要作主语或宾语,而后面的句子是完整的,不缺主语和宾语,所以不用that.
而下面这句就要用that了,This is the room that we lived in last year.