同学你好，该知识点来自沪江网校《英语零基础直达PETS三级/公共英语三级【年中特惠班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学，你好！

这里it就是主语，不是虚主语。it作主语，可表示天气、时间、距离、度量衡以及环境等。例如：
It is fine today.
-What time is it? -It's eight o'clock.
I's a half hour's ride to our school.
It is ten kilometers from the city to our hometown.
It was very quiet at that moment.

如果是虚主语，是可以放回做主语，那这句是Eight o'clock is.这句话有问题的，不可以be结尾哦

希望助助的解答可以帮助到同学，祝同学学习进步~~