A1
が — the subject, and new information
How が marks the subject in Japanese, when it must be used instead of は, and the new-versus-known distinction that separates them.
が marks the grammatical subject. Where は says what we are talking about, が says which one.
猫がいます。
Neko ga imasu.
There’s a cat.
雨が降っています。
Ame ga futte imasu.
It’s raining.
New information takes が, known information takes は
This is the distinction worth holding on to. Something being introduced for the first time is が; once it is established and you go on talking about it, it becomes は.
昔々、おじいさんがいました。おじいさんは山へ行きました。
Mukashi mukashi, ojiisan ga imashita. Ojiisan wa yama e ikimashita.
Long ago there was an old man. The old man went to the mountain.
Every Japanese folk tale opens exactly like this, and the particle switches in the second sentence. He arrives as news, and then he is what the story is about.
Question words always take が
Because a question word is asking which one — the definition of new information. It can never be the topic, since you cannot already be talking about something you are asking to identify.
誰が来ましたか。
Dare ga kimashita ka.
Who came?
誰は来ましたか。
誰が来ましたか。
誰 is the thing being asked about, so it cannot be marked as already known. The answer takes が too — 田中さんが来ました.
Some things simply take が
A group of adjectives and verbs mark what English would call their object with が rather than を: 好き, 嫌い, 上手, 下手, ほしい, わかる, できる.
日本語がわかります。
Nihongo ga wakarimasu.
I understand Japanese.
猫が好きです。
Neko ga suki desu.
I like cats.
好き is an adjective — likeable — so 猫が好き is closer to cats are likeable than to I like cats. Once you see the shape, the particle stops being an exception and starts making sense.