Japanese learners often understand the particle を as a direct object marker, but later, in their study, they find it used in a strange context, which cannot be explained with that use. In this post, I decided (briefly) to list those and explain them in a few words.
But first, a short historic note. The hiragana を probably comes from kanji 遠 written in the cursive sōsho style. The original spelling was ‘wo’, which eventually changed to ‘o’, however, historic spelling has been preserved just like in the case of the particle ‘は’ and particle ‘へ’.
Well, let’s move the usages:
1) expresses direct object with transitive verbs:
毎日（まいにち）カレーを食（た）べるの？
Do you eat curry every day?
私（わたし）たちは日本語（にほんご）を勉強（べんきょう）している。
We learn Japanese every day.
昨日（きのう）、ニュースを見（み）ました。
I watched the news yesterday.
Because only one particle を can be used in the clause (part of the sentence), with causative form (to make someone do something) the second を indicating a person that is made to do something is replaced with に.
あなたの犬（いぬ）をタブレットを食べさせる。(WRONG)
あなたの犬にタブレットを食べさせる。(OK)
Feed the tablet to the dog. (Make the dog eat the tablet.)
2) With intransitive verbs expressing movement (歩く 飛ぶ 登る 泳ぐ) it expresses what we are moving along, over, or through.
北極(ほっきょく)を泳（およ）ぐなんて普通（ふつう）じゃないことです。
Swimming across the north pole is not something ordinary.
広（ひろ）い空間（くうかん）を歩（ある）いている。
To be walking across wide, open space.
険（けわ）しい山道を登（のぼ）るには、はじめはゆっくり登（のぼ）らなくてはいけない。
In order to climb steep hills, you have to go slowly at first.
(By the way, when particle に is used with 登る there is a nuance difference, for example:
山に登る
山を登る
it indicates going up, to the peak, while using を indicates just going up the mountain, not necessarily aiming for the top)
3) Marking place where some movement stars, ‘away’, ‘from’, ‘off’.
アムステルダムを出（で）た。
I am going away from Amsterdam. (I am leaving Amsterdam.)
席（せき）を立（た）つ
To leave one’s seat.
ただ列車（れっしゃ）を降（お）りたの？
Have you just left the train?
オランダを離（はな）れた。
I left the Netherlands.
4) を can also mark a cause with verbs expressing emotion like 悲（かな）しむ、喜（よろこ）ぶ and 怒（おこ）る。
メアリーは休暇（きゅうか）でアメリカに帰（かえ）れないのを悲（かな）しんでいた。
Mary was very sad about not going back to US for the holidays.
彼（かれ）はその贈り物（おくりもの）を喜（よろこ）んだ。
He was pleased with that gift.
あたしのことを怒（おこ）っているの？
Is he mad because of me?
5) You might also encounter it in construction conditional ば（たら・と）ものを, where it means ‘but/although’. The を in it expresses exclamation/concession, functions of を from classical Japanese.
「もっと早はやくタバコをやめれば健康問題けんこうもんだいなしに済すんだ**ものを**、手遅ておくれになってしまった。」
“If I had quit smoking much earlier I would have ended up without any health problems, **but** now it is too late.”
トニー：「まったく、ボリス、おとなしく死（し）んでおればいい**ものを**…」
Tony: “Dear God, Boris, it would have been good if you had just shut up and died, **but…**”
By the way, there are cases where particles が and を are both accepted.
For example with auxiliary たい (to want):
ハンバーガーを食べたい。
ハンバーガーが食べたい。
I want to eat a hamburger.
According to some, が indicates the stronger feeling of ‘wanting’.
With potential form:
redittorさんは日本語（にほんご）が/を読めます。
Redditor can read Japanese.
With 好き (many people are surprised that it is allowed):
ホロさん、どうしてローレンスがあなたを好（す）きか分（わ）かる
Holo, I can see why Lawrence loves you.
Well and that’s all folks 🙂 I believe that をwon’t surprise you anymore.
If you are curious, you can follow me on Reddit to get info about new posts 🙂
I am mrnoone, and this was briefjapanese.
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