你的句子是宾语从句.it是形式宾语,不能省略.真正宾语是when they talk with their mouths full of food.
注意下面的句子不能有it
Do you mind (it在这里不能存在）if / when I go with you?---------因为这是条件/ 时间状语从句.
现在你明白了吗?那Do you know it where she was born？为什么不对 这总是宾语从句吧Do you know it where she was born？--------是错句，多了一个it.这是宾语从句， where she was born是know的宾语，而it是多余的，并且不能充当形式宾语。明白了吗？