Used before nouns and noun phrases that denote a single but unspecified person or thing.
예문
1. She has such a wonderful voice. (그녀는 목소리가 썩 좋다) 2. take a rest (쉬다) 3. A heavy rain began to fall. (폭우가 내리기 시작했다) 4. A friend of mine in the United States called me yesterday. (미국에 있는 내 친구 하나가 어제 내게 전화를 했다) 5. once a day (하루에 한 번)
