形式主语和主语从句怎么区别？谢谢回答~~！！

Question

____ is Britain now used to be three different countries and over many years the three countries became one.
A It B What C Where D As
答案是B 为什么不选A?

Answer 1

关键要理解Used 是谓语，而不是过去分词，在这里表示“曾经是”。 如果it作形式主语，则应在used前面加that，即： it is Britain now that used to be three different countries and over many years the three countries became one

如果没有that，而简单地变成：it is Britain now used to be three different countries and over many years the three countries became one，则used就成为过去分词，而Britain就要成为被动主语，显然这不符合语义。

Answer 2

主语从句是一个句子，这个从句在句子中的性质是起主语作用。
形式主语是it

是个代词，指代了XXX