“三单”是一般现在时中“第三人称单数”的缩写,也就是指“he,she,it,或者单个的人名,以及可数名词的单数或者不可数名词。”
根据语法规则,在一般现在时中,如果主语是第三人称单数,那么谓语动词要加-e/-es,如:
He goes to school every day.
She always gets up early in the morning.
It rains a lot in summer in China.
Jim plays football on Sundays.
An apple falls off the desk.
Water flows from the top of the mountain.
参考资料:个人总结,供参考