It was …when…是状语从句，It was…that …是强调句。结构是It is (was) 被强调部分+ that (who) + 句子其他部分。比如：
It was seven o'clock when we reached the station.（状语从句）我们到达车站时是七点钟。
=It was "at" seven o'clock "that" we reached the station.(强调句，定语从句）我们是在七点钟到达车站的。
强调句和时间状语从句的区别是：时间状语从句可以去掉，不影响句子的完整性。而强调句that/who引导的句子是不能去掉的，否则句子就不完整了，单独的“It was at seven o'clock”是错误的。
时间状语从句和定语从句的区别其实跟上面说的是一样的，强调句型其实就是一种特殊的定语从句。
本身最简单的表达法是：We reached the station at seven o'clock.然后为了强调"at seven o'clock"，就变成了强调句It was "at seven o'clock" that we reached the staion，同时它也是定语从句：从句“that we reached the staion”修饰的是it=the time.
扩展资料：
注意状语从句的动词和主句动词之间的配合。
When I gotthere, Jack had left.（杰克离开在前，故用过去完成时）
我到那的时候，杰克已经离开了。
While I was sittingin the sun, my wife was working in the gardem.（while 多用于两个平行进行的动作）
当我妻子在做饭的时候，我们在晒太阳。
参考资料来源：百度百科-时间状语从句