Learn Hardcore Russian: Я положил книгу на стол.

Because книгу is the direct object of the verb положил. In Russian, transitive verbs take their objects in the accusative. Feminine nouns ending in -а change that ending to -у in the accusative singular.

Why is it на стол, not на столе or some other form?

Here на стол uses the accusative because it indicates movement onto a surface (“onto the table”). If you wanted to say “on the table” as a location without movement, you’d use the prepositional case: на столе.

Why aren’t there any words equivalent to “the” or “a” before книгу or стол?

Russian has no articles—definite or indefinite. Nouns stand alone, and context or word order conveys definiteness or indefiniteness.

What aspect is the verb положил, and why is that important?

положил is the past tense of the perfective verb положить, which denotes a completed action (“put down, placed”). Its imperfective counterpart is класть (present я кладу, past я клал/кла́ла). Use the imperfective for ongoing, habitual, or repeated actions.

Why do we use and not ? They both mean “put,” right?