が (ga): subject marker and the wa vs ga trap
が marks the strict grammatical subject. Critical difference with は: は says "here's what we're talking about", が says "here's who does the action or holds the quality".
Its base function
が introduces the subject, especially when it's new info or the answer to a "who/what" question.
雨が降っています。
あめ ga ふって います。
It's raining (rain is falling).
The focus is on what's falling: it's rain, not snow.
何がありますか？
なに ga ありますか？
What is there?
With "who/what" questions, always が, never は.
Verbs and adjectives that require が
Some verbs and adjectives force their complement to be marked by が, not を. Memorize as a block:
- ある / いる
- Existence (objects / living beings).
- 分かる (わかる)
- To understand, to know.
- できる
- To be able, to know how to.
- 好き (すき) / 嫌い (きらい)
- To like / to dislike.
- 上手 (じょうず) / 下手 (へた)
- Good at / bad at.
- 欲しい (ほしい)
- To want, to desire.
日本語が分かります。
にほんご ga わかります。
I understand Japanese.
You say 日本語が分かる, never 日本語を分かる. Hard rule.
The classic trap: は vs が
私は田中です。 (わたし は たなか です)
"As for me, I am Tanaka." Standard introduction. The topic (私, わたし) is the frame, we comment on who I am.
私が田中です。 (わたし が たなか です)
"I am Tanaka." Out of a group, I'm the one named Tanaka. Answer to "who is Tanaka?".