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.