The particle "が" is used to mark the grammatical subject, especially when introducing new information or answering 'who/what' questions. It is also used with adjectives of desire (〜たい), ability (できる), and existence (ある/いる). The use of "が" emphasizes the subject and indicates that it is new or unexpected. For example, in the sentence "田中さんが来ました。", the particle "が" emphasizes that it was Mr. Tanaka who came, rather than someone else. In the sentence "日本語ができますか。", the particle "が" indicates that the subject is the ability to speak Japanese.