判断是否单三的条件——时态——一般现在时，现在完成时等主语——第三人称单数做主语(he/she/it/Mary/the boy/that dog)谓语动词——单数第三人称形式例如Jack often goes to school by bus.It has two eyes.She has been to Beijing three times.
不是你/我/我们/你们的非复数形式
