请认真回答1.A new school was put up (where) there had once been a

请认真回答
1.A new school was put up (where) there had once been a temple.(there可以作主语吗)
A new school was put up ___used to be an old temple 怎么填空?有几种填法?
2.It is 50 years __ the PRC was founded.为什么不是after
3..The volleyball match will be put off if it (rains)如果用虚拟语气不是该用rained吗 用来表将来?
4.Remenber to send me a postcard next time you( write) to me.为什么这也是主将从现?
第一个问题的第一句话对吗