Learn Hardcore Russian: Куда ты положил книгу?

куда is an interrogative adverb meaning “to where?” or “where to?” It asks about the direction or destination of an action. By contrast, где means “where?” in the sense of a location or place (i.e. “at what spot?”). You choose куда when you want to know where something was moved, not just where it currently is.

Is куда a preposition? Do you need another preposition after it?

No, куда is not a preposition but an interrogative adverb of place. It stands on its own and already carries the meaning “to where?” You do not add another preposition after куда. For example, you say “Куда ты положил книгу?”, not “На куда ты положил книгу?”

Why is книгу in the accusative case?

книгу is the direct object of the transitive verb положил (“put”). In Russian, direct objects of verbs of action normally take the accusative case. Here you’re asking what was placed (the book) and it must be in accusative: книга → книгу.

What aspect is the verb , and how would using the imperfective change the meaning?