を (wo): the direct object particle
を is the most stable of the five particles: a transitive verb, an object, a を. But its pronunciation and a secondary use with motion verbs deserve a look.
The second pronunciation trap
It's written を but pronounced "o" in everyday Japanese. The initial w has all but vanished, except in singing or slow reading. It's the second of three particles with a special pronunciation, after は (read "wa") and before へ (read "e" when it marks direction).
Base use: direct object
If a verb can take an object (transitive), that object is marked with を. Ultra-stable rule, no exception in N5.
本を読む。
ほん o よむ。
Read a book.
本 (ほん, book) is the object of 読む (よむ, read). Marked by を.
コーヒーを飲む。
コーヒー o のむ。
Drink coffee.
音楽を聞く。
おんがく o きく。
Listen to music.
The lesser-known use: crossing space
With motion verbs like 散歩する (さんぽする, take a walk), 渡る (わたる, cross), 歩く (あるく, walk) or 飛ぶ (とぶ, fly), を no longer marks an object but the space traversed by the motion. Subtle but common.
公園を散歩する。
こうえん o さんぽ する。
Take a walk through the park.
The park is what one passes through while walking, not an object being "walked".
橋を渡る。
はし o わたる。
Cross the bridge.