首先来介绍一下some的用法：
1、some 与不可数名词连用：一些,若干
例句：There's some ice in the fridge.冰箱里有些冰块儿.You left some money on the table.你把钱落在桌子上了.Please have some cake.请吃点儿蛋糕.
2、some 与复数可数名词连用,通常指三个或三个以上
例句：If you put some pictures on the wall,the room will look brighter.你要是在墙上挂几幅画,这个房间就更有生气.
3、some 与单数可数名词连用：未知的或未确指的人、地点或事物
例句：Some man at the door is asking to see you.门口有人要见你.She won a competition in some newspaper or other.她参加某报纸举办的比赛获胜.
所以对于你的问题的回答是：some可以修饰单数名词；肯定、否定句并无硬性规定.