Particles が, は, を, と all define direct object in sentences from English’s perspective, but they work differently.
を defines a direct object as the target of some action:
- 私は新しい靴を買った -> I bought new shoes
- 彼は大学で経済を勉強している -> He studies economy at college
- 彼女は本を読んでいる -> She is reading a book
- 彼は車を洗っている -> He is washing his car
- 話題を変えよう -> Let’s change the subject
が is used when the object has an effect on the subject, instead of subject actively working on the object. For instance が defines an object of preference, desire, etc..
- 彼女は子供が好きだ -> She likes kids
- 私は酒が嫌いだ -> I don’t like alcohol
- 新しい車が欲しい -> I want a new car
- ビールが飲みたい -> I want to drink beer
- 彼と話がしたい -> I want to have a conversation with him
However some verbs of preference use を instead of が. For instance 好む is translated “like”, but it is structured like “have an affinity toward something”, thus the object is the target of an outward action:
- 彼は孤独を好む -> He likes solitude
- 絹は湿気を嫌う -> Silk hates, is adversely affected by, moisture
- 彼女は苦労を厭わない -> She doesn’t mind hard work
- うちの子供は風呂に入るのを嫌がる -> My kid hates taking a bath
As always, it is best to learn through live examples rather than trying to theorize. Actually in some cases the difference between が and を is not very clear, and you can use either them.
- ビール が/を 飲みたい -> I want to drink beer (either が or を is OK)
- 彼と話 が/を したい -> I want to have a conversation with him
- 私はどうも彼 が/を 好きになれない -> I somehow can’t warm up to him
- 野球が好きだ -> I like baseball (canNOT use を)
が is used for an object of ability
- 彼は仕事が出来る -> He can do his work. He is very capable
- 彼女は料理が上手い -> She is good at cooking
- 答えが分からない -> I cannot figure out the answer
However note that you can use を when you use slightly different sentence structures. The below can all be translated as “he can speak Korean”:
- 彼は韓国語が話せる -> He can speak Korean
- 彼は韓国語を話す -> He speaks Korean
- 彼は韓国語を話すことが出来る -> He is capable of speaking Korean
は is used when you make a comparison/contrast, even though the focus is still on the identity of the object.
- 今日はパンは買わなくても良い -> I don’t need to buy bread today (but I have to buy other stuff)
- 僕はサッカーはよく観るが野球は殆ど観ない -> I watch football often but seldom watch baseball
- 準備はちゃんとしたが自信は無い -> I did prepare well, but I still don’t have confidence
と is used to create an object clause for a verb.
- 私は自分が間違っていると思わない -> I don’t think that I am wrong
- その値段は高過ぎると感じた -> I felt the price was too high
Also note that many active-voiced sentences in English are expressed in passive voices in Japanese, thus objects in English become subjects in Japanese.
- 失敗は避けられない -> Mistakes cannot be avoided. You cannot avoid making mistakes
- 鍵が見つからない -> The key cannot be found. I cannot find the key