Subject Particle: が (The grammatical actor and new information)

Mental Model: The Laser Pointer (Inward Focus)

While topic は creates a wide, sweeping stage and looks forward to the commentary, subject が acts like a sharp laser pointer directed straight inward at the noun:

• It answers the question: “Exactly WHO or WHAT is responsible?”
• It isolates and spotlights the specific actor out of an entire group: 私が 行きます。(わたしが いきます。) = “I am the one who will go (not anyone else!).” 田中さんが 勝ちました。(たなかさんが かちました。) = “Tanaka is the one who won.”

Function 1: Reporting Brand-New Sensory Information (Phenomenon Sentences)

When you look out the window and observe an unexpected event unfolding in real time, you are not setting a philosophical topic—you are reporting raw sensory input. Japanese uses が to announce brand-new information entering the scene:

• あ、雨が 降ってきた！(あ、あめが ふってきた！) (“Look, it started raining!”)
• バスが 来ましたよ。(バスが きましたよ。) (“The bus is here!”)
• 桜の花が 咲きました。(さくらのはなが さきました。) (“The cherry blossoms have bloomed.”)

If you used は (*雨は降ってきた), it would sound like you were expecting the rain as an established topic of conversation.

Function 2: Pairing with Question Words (The Interrogative Rule)

This is one of the most reliable rules in all of Japanese grammar: Question words representing unknown actors MUST take が, never は.

Because question words (誰(だれ), 何(なに), どれ) represent unknown information, they cannot serve as a shared topic: • Question: 誰が 来ましたか。(だれが きましたか。) (“Who came?”) • Direct Answer: 田中さんが 来ました。(たなかさんが きました。) (“Tanaka came.”) The question asks for the actor (誰が), so the answer mirrors it with laser focus (田中さんが).

Function 3: Verbs and Adjectives that Grammatically Require が

In English, verbs like “to understand” or adjectives like “to like” take direct objects. In Japanese, they represent natural states or spontaneous conditions, and therefore demand が:

• 1. Existence (ある / いる): あそこに 猫が います。(あそこに ねこが います。) (“There is a cat over there.”) 時間と お金が あります。(じかんと おかねが あります。) (“I have time and money.”)
• 2. Potential & Ability (できる / 分かる): 日本語が 分かります。(にほんごが わかります。) (“I understand Japanese.”) ピアノが できます。 (“I can play the piano.”)
• 3. Desires & Preferences (好き / ほしい): 猫が 大好きです。(ねこが だいすきです。) (“I love cats.”) 新しいパソコンが ほしいです。(あたらしいパソコンが ほしいです。) (“I want a new computer.”)

Function 4: The Actor in Subordinate Clauses

When a sentence contains a smaller descriptive clause modifying a noun, the subject of that sub-clause almost always takes が:

• 私が 撮った 写真(わたしが とった しゃしん) (“The photo that I took”).
• 母が 作った 料理(ははが つくった りょうり) (“The food that my mother made”). Inside a relative clause, using は would improperly hijack the topic of the entire overarching sentence.

Foreigner Traps to Avoid

• Trap 1: Asking 誰は: Never ask *誰は来ましたか. Question words cannot be topics; always use 誰が.
• Trap 2: Forgetting が with あります / います: The entity existing is always marked with が: ペンがあります (never *ペンをあります).