四个定语从句.This is the school.My mother works in the school用定语从句

四个定语从句.
This is the school.My mother works in the school用定语从句改成一句,是不是改成This is the school in which my mother works
还有I still remember that day.You left for Beijing on that day.合并好像是I still remember that day on which you left for Beijing.
I will never forget the time.I spent the time with my grandparents.用定语从句合并.想不好用which,who,where,when,why了.答得好奖财富值.
Could you explain the reason?You were late for the reason.用定语从句改写为一句是不是Could you explain the reason for which you were late?