A1
を — the object
How を marks the direct object in Japanese, why it is read “o”, and the verbs that take it where English would not.
を marks what the verb does something to. It goes immediately after the object, and the verb comes at the end.
パンを食べます。
Pan o tabemasu.
I eat bread.
日本語を勉強します。
Nihongo o benkyō shimasu.
I study Japanese.
Written を, said o. The kana exists for essentially nothing else — it is the particle and almost never a syllable inside a word, which is why it is the one hiragana many learners can read before they can write.
Movement through a space
Its other job, and one English has no marker for: を also marks the space a movement passes through or leaves.
公園を散歩します。
Kōen o sanpo shimasu.
I walk through the park.
電車を降ります。
Densha o orimasu.
I get off the train.
Neither park nor train is being acted on. They are the space the movement crosses, and Japanese uses the same particle.
Not every English object takes を
A group of words takes が instead: 好き, ほしい, わかる, できる. Those are adjectives or intransitive verbs in Japanese even where English makes them transitive.
日本語をわかります。
日本語がわかります。
わかる is closer to be understandable than to understand, so the language is its subject rather than its object. 日本語がわかります is Japanese is comprehensible to me.
One を per clause
A clause takes at most one を. Two is a reliable sign the sentence needs splitting or a different verb — and it is one of the few hard constraints in Japanese particle use.