N + 을/를
Appending 을/를 to a noun denotes the object of the sentence.
When the noun ends in a consonant, the ending 을 is used.
When the noun ends in a vowel, 를 is used.
The object particle is usually used to indicate specifically which noun in the sentence is being acted on by an action verb, but just like the subject particle, if the meaning is obvious then it isn’t needed. Generally, if the object noun is farther away from the verb in the sentence, and its not clear what object the verb is acting on, then it is used.
어제 사과를 먹었어요? = Did you eat the apple yesterday?
어떤 노래를 좋아해요? = Which song do you like?