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 положил and not поставил? They both mean “put,” right?

They both translate as “put,” but with a nuance: положить implies laying something flat, while поставить implies setting something upright. Since a book usually lies flat, положил is more natural.

How is the past tense положил formed, and why does it end in -л?

Russian past tense for perfective verbs is formed by taking the infinitive stem (положи-) and adding -л for masculine singular: положил. The ending agrees in gender and number with the subject. A female speaker would say положила, plural would be положили.

What does the prefix по- in положил do?

The prefix по- adds the perfective aspect and often a sense of “briefly” or “once” for placement verbs. It turns the imperfective root (here irregularly класть) into a perfective form (положить).

Why is the imperfective counterpart not ложить, but класть?

This is an irregular verb pair in Russian: the perfective положить corresponds to the imperfective класть, not ложить. Such root alternations are historical and must be memorized.

Is the word order rigid? Could I say Книгу я положил на стол?

Russian has flexible word order for emphasis. The neutral order is Я положил книгу на стол, but fronting книгу (as in Книгу я положил на стол) emphasizes the object. The meaning remains the same.

How would I say “I am putting the book on the table” (an ongoing action) instead of “I put the book on the table”?

Use the imperfective present tense: Я кладу книгу на стол. If you want a future imperfective (“I will be putting…”), you’d say Я буду класть книгу на стол.