Learn Hardcore Korean: chaeksangeseo chaegi tteoreojyeosseoyo.

에서 marks the location from which or at which an action occurs. Since the book “fell off” the desk (i.e. from the desk), 책상에서 is appropriate. In contrast, 에 alone would simply indicate a static location (“at the desk”), not “from the desk.”

What role does the particle 이 play in 책이?

이 is the subject marker attached to nouns ending in a consonant. Here, 책이 marks “the book” as the grammatical subject—the thing that performs the action of falling. (If the noun ended in a vowel, you would use 가 instead.)

Why isn’t 책 marked with the object marker 을/를?

The verb 떨어지다 (“to fall”) is intransitive, meaning it doesn’t take a direct object. The book isn’t being acted upon by someone; it’s performing the action of falling on its own. Therefore, you use the subject marker (이) rather than an object marker.

What’s the difference between 떨어지다 and 떨어뜨리다?

떨어지다 (intransitive): “to fall” (something falls by itself)
떨어뜨리다 (transitive): “to drop” (someone drops something)
Example:• 컵이 떨어졌어요. → The cup fell.• 제가 컵을 떨어뜨렸어요. → I dropped the cup.