- particle indicating the direct object of a verb
Usage notes 
Like other Korean particles, 을 (eul) is a postposition. It marks the preceding noun as the direct object of the following verb.
을 (eul) only comes after a word ending in a consonant. If the modified word ends in a vowel, 를 (reul) is used instead:
을 (eul) is optional if it can be inferred that the noun is the direct object (as opposed to the subject).
If the noun is the main topic of the sentence and it can be inferred that the noun is the direct object (as opposed to the subject), 은 (eun) is usually used instead.