Question

____ is Britain now used to be three different countries and

Answer 1

主语从句,what is Britain now作主语.你可能有点混淆it作形式主语时的区别.举个例子

主语从句---What I said was important.（翻译：我所说的很重要）.慢慢体会what放在句首的味道.可以反过来读 I said what（当然英语中没这种说法,是句子成分划分时才用,主语I 谓语said 宾what）.what I said可以看成一个句子,可以在心里悄悄的当做It（形式主语it） ,此话就简单为it is important.简单吧?

另外有些特殊句式必须用It作主语.It is +adj+ to do sth

It is important to learn English well.

建议：英语多读课本多背书,读到一定境界直接就知道.无需太花功夫纠缠语法.