it是形式賓語(yǔ),真正的賓語(yǔ)是when people talk with their mouths full,因?yàn)閔ate在此處是及物動(dòng)詞,后面必須加賓語(yǔ),但是when people talk with their mouths full是一個(gè)完整的句子,不能做賓語(yǔ),就必須用it充當(dāng)形式賓語(yǔ).
這個(gè)時(shí)候when people talk with their mouths full雖然是一個(gè)疑問(wèn)句,但是卻是陳述句的形式,when翻譯成“當(dāng)……的時(shí)候”.
when做“當(dāng)……的時(shí)候”講的時(shí)候,不一定就是引導(dǎo)時(shí)間狀語(yǔ)從句,它可以用于從句中.比如what：
I don't believe what you said.
這里的what同樣是疑問(wèn)詞,也作“什么”講,但是它僅僅是從句的一個(gè)成分而已（賓語(yǔ)）.