Some students think it cool to have their hair dyed yellow.为什么用it 不用that 高三狗求指教

it是think的宾语，又是表语cool的主语，代替长的to不定式放表语cool前面，避免头重脚轻。

it 在这里做think的形式宾语， 后面cool为宾补，to have的部分是think的逻辑宾语
that 不能这样用

it是宾语啊，cool是它的补语，你要是想用that的话，那得换成that引导的宾语从句
Some students think that it is cool to have their hair dyed yellow.

it 指代 to have their hair yellow 可以说为it is cool to have their hair dyed yellow
that 没有这个用法吧

一般在一个句子中（前后句中间有标点的不算）,it 较多用作代词,that较多用于引导从句.
这里 it 用于指代 to have their hair dyed yellow 这件事