が

The particle が has several key roles in Japanese:

1. Marking the Grammatical Subject
Its most basic job is to show who or what is doing the verb. In the sentence 「猫が寝ている」 (The cat is sleeping), が tells us the cat is the one sleeping.

2. Reporting New Information (Sensory Report)
When you see something happening for the first time and just report it, you use が.
Example: 「雨が降っています」 (It is raining). You aren't talking 'about' the rain as a topic; you are reporting the event you see.

3. Exhaustive Listing (The 'Focus' Function)
When が follows a noun, it often implies 'this one and no other.' If someone asks 'Who is the teacher?', the answer 「私が先生です」 means 'I am the one who is the teacher.'

4. Stative Predicates and Existence
In Japanese, the things you like, understand, want, or are able to do are often marked with が rather than を. For words like 好き (Like) / 嫌い (Dislike) and ほしい (Want), this is because they are actually adjectives (な-adjectives and い-adjectives), not verbs, and adjectives inherently cannot take the direct object marker を. Furthermore, words like わかる (understand) and potential forms (られる/える) are stative verbs (verbs describing a state rather than an action), which is why they also take が instead of を. This also applies to verbs of existence (ある / いる), which take が to mark the thing that exists (e.g., 時間がある, 猫がいる). This can sometimes lead to a 'Double が' structure where two が particles appear in the same sentence (e.g., 私が車が欲しいです). However, using が for the first noun forces an 'Exhaustive Listing' meaning ('I am the one who wants a car'). For a neutral statement, native speakers use the topic marker は instead (e.g., 私は車が欲しいです). Note that in modern spoken Japanese, を is also commonly used with potential forms and the verb suffix たい (want to do). This category includes:

• 好き・嫌い (Like/Dislike)
• わかる (Understand)
• ほしい (Want)
• たい (Want to do)
• Potential forms (Can do)
• ある / いる (To exist)

5. Question Words
When a question word like 誰 (who) or 何 (what) is the subject, it must be followed by が, never は. (Note that question words can take other particles like を or に when they serve other roles, but they can never be the topic marked by は).

6. Subordinate Clauses
In complex sentences, が is the standard marker for the subject within a subordinate or relative clause (e.g., 「私が買った本」 - The book that I bought). Using は in these clauses can change the meaning or sound unnatural. Additionally, が can often be replaced by の in these relative clauses (e.g., 私が買った本 -> 私の買った本).

7. Conjunction (But/However/Background)
Beyond marking subjects, が is also frequently used at the end of a clause. While it can mean 'but' or 'however' (が) as in 「行きたいですが、時間がありません」 (I want to go, but I don't have time), it is also frequently used to provide background context or as a soft introductory remark without a contrastive meaning (e.g., 「すみませんが、質問があります」 - Excuse me, I have a question).

8. Contrasting は and が (Old vs. New Information)
A fundamental difference between the topic marker は and the subject marker が is how they handle information. は is used for known or old information (the topic we are already talking about), while が introduces new information. For example, if someone asks 'Where is the cat?', you answer 「猫はあそこです」 (As for the cat, it is over there) because the cat is already the topic. But if you suddenly spot a cat, you say 「あ、猫がいます！」 (Look, there is a cat!) because the cat is new information being introduced to the conversation.