Question

i think __important for us to learn english well

填 it 。为什么呢？
that's为什么不可以？

Answer 1

i think 【it】important for us to learn english well

it 是形式宾语，真正的宾语是后面的不定式 to learn english well

实际上句子是：

i think 【 to learn english well】important for us。

如果要改成that从句，那应该是 ：

i think 【that  it is 】important for us to learn english well

Answer 2

that's不可以 that it is可以 因为用that是引导从句 从句总得有个主语吧