that可以代指一件事,而it主要代指物体.
第二题是宾语从句.用it指代后this that it 都可以直接作主语
都可以做代词
指代事物
例子：this /that/it is an apple.
但是,只有it可以做形式主语
例：it is better to say hello to him.
it 是形式主语,真正的主语是to say hello to him.
it作为形式主语的时候可以代替动词不定式/ing/一个句子.
面的 when and where the meeting world would be held.