In spanish what's the difference between direct object pronouns and indirect object pronouns

Direct object pronouns are used to replace direct object nouns, but indirect object pronouns are used to replace nouns that act as indirect objects. Also, indirect object pronouns answer to questions such as: to whom? or for whom? about the verb, but direct object pronouns answer to questions such as: whom? or what? about the verb

Deberías estudiar esta noche

How is this a spanish question?