を

を (pronounced "o," not "wo," in modern Japanese) marks the direct object of a transitive verb: the thing being eaten, read, drunk, made. It also marks the path traversed by a motion verb (公園を歩く — "walk through the park") and the point of departure from an enclosed space (家を出る — "leave the house").

The most common beginner mistake is using を with stative predicates that look transitive in English: "I like sushi" is 寿司が好き, not 寿司を好き — 好き is an adjective, not a verb, and adjectives don't take direct objects. Same with わかる, できる, ある: the apparent "object" gets が.

Note: を is almost never doubled in the same clause. A sentence can't have two を-marked nouns; if you need two "objects" the second is usually expressed with に or で.