i donts have a sister还是i dont have sister

i donts have a sister正确。你的问题牵扯名词表类别问题。a/an加可数名词单数A cat is a domestic animal。猫是家养动物零冠词加可数名词复数Cats is domestic animals。the表示人或事物的类别The cobra is dangerous。眼镜蛇是危险的If you park your car in the worng place,a traffic policeman will soon find it。一旦你把汽车停错了地方,交警很快就会发现。