不是代词.是100%的疑问副词.
通常,你的老师总是告诫你,副词前千万不要加任何介词,这句话在多数时候是对的,但不总是对的.凡事皆有例外.
一般情况下,at/on/ to + adv.是错误的.
但介词from+ adv.是对的.
我们不可以说：Where are you going to?
而说：Where are you going?
但却必须说：Where are you from?(from where is right.)
再如,从国外回来：
come back / return from abroad (adv.)
但不可以说:study in abroad; go to abroad.