The particle "が (ga)" in Japanese is primarily used to indicate the subject of a sentence. It highlights the noun that performs the action or is in a certain state. For example, in the sentence "猫 (neko) が 走る (hashiru)," "が" shows that the "猫" is the one running. Additionally, "が" can express contrast or emphasize a specific subject in a sentence. For instance, in "彼 (kare) が好きですが、彼女 (kanojo) は嫌いです," it emphasizes that "I like 彼" but not "I like 彼女." This usage helps clarify relationships between subjects and actions.