が

が marks the grammatical subject — the noun that performs the verb, exists, or carries the property. It is also the default marker on the subject of stative predicates (好き, わかる, できる, ある, いる), where English speakers expect an object: 寿司が好きです — "[I] like sushi," literally "[as for me,] sushi is liked."

The contrast with は is the core beginner puzzle. が introduces new information or identifies a specific referent; は presupposes the referent and comments on it. The textbook test: a "wh-" question takes が in both the question and the answer (誰が来ましたか — 田中さんが来ました). Once the referent is established, follow-up sentences flip to は.

Secondary use: が is also a contrastive conjunction ("but") between clauses — 高いですが、おいしいです — but that が is a different word from the case particle, even though it's written the same way.