It is the first / second time.... that....是什么丛句

主语从句，it为形式主语，不过已经固定成了句型了。
itis the first time that sb. have done sth.
这是某人第一次做某事
it was the first time that sb. had done sth.
that 从句中要用完成时

It is the first / second time.... that....是主语从句。
that 后面的应该是主语， it 是形式主语。

同位语从句