を
を
particle
direct object marker
1.
direct object marker
Marks the direct object of a transitive verb—the thing that receives the action. 本を読む (read a book), ご飯を食べる (eat rice/a meal).
2.
space traversed marker
Marks the space through which movement occurs with verbs of motion. 道を歩く (walk along the road), 空を飛ぶ (fly through the sky), 橋を渡る (cross the bridge).
3.
point of departure marker
Marks the starting point or place from which one leaves with verbs like 出る (leave), 卒業する (graduate from). 家を出る (leave the house), 大学を卒業する (graduate from university).
を is exclusively used as a grammatical particle—it doesn't appear in any other words in modern Japanese. In casual speech, を is sometimes dropped: ご飯食べた (ate a meal). The three main uses are: (1) direct object, (2) space traversed with motion verbs, (3) point of departure with leaving verbs.