が for identificational focus

When the subject of a sentence is being singled out — as the contested element, the answer to a wh-question, or new information being introduced for the first time — Japanese marks it with が rather than は. Often called 'exhaustive listing,' 'identificational,' or 'focus' が. Contrasts with は, which backgrounds the subject as a known topic. 誰が来た？ ('who came?') gets answered 田中さんが来た ('it was Tanaka who came'); 雨が降っている ('it's raining') introduces the rain as new.