I hate it when people talk with their mouths full.

it是形式宾语,真正的宾语是when people talk with their mouths full,因为hate在此处是及物动词,后面必须加宾语,但是when people talk with their mouths full是一个完整的句子,不能做宾语,就必须用it充当形式宾语.
这个时候when people talk with their mouths full虽然是一个疑问句,但是却是陈述句的形式,when翻译成“当……的时候”.
when做“当……的时候”讲的时候,不一定就是引导时间状语从句,它可以用于从句中.比如what：
I don't believe what you said.
这里的what同样是疑问词,也作“什么”讲,但是它仅仅是从句的一个成分而已（宾语）.