see sb doing sth 是句型。
意思是：“看见某人正在做某事” （动作正在进行）
如：
I saw a group of boys playing basketball on the playground when I looked out of the window.
当我往窗外看时，我看见一群男孩正在操场上打篮球。
区别：
see sb do sth 看见某人做了某事（动作已结束）
I often see him play basketball on the playgroung.
我经常看见他在操场上打篮球。
拓展（类似用法的常见面的动词还有）：
hear sb do/doing sth 听见某人做/在做某事。
如：
1). I often hear her sing a song .
我经常听见她唱歌。
2). I hear her singing a song in the next room now.
现在我听见她在隔壁唱歌。
2. feel ... do / doing sth .感觉……做/在做某事。
如：
1). I feel my pulse beating .
我感觉我的脉搏在跳动。
2). She felt him hear of it.
她感觉他听说了这事。
3. watch sb do / doing sth 看见某人做了/正在做某事。
如：
1). I watched him steal that book.
我看见他偷了那本书。
2). He likes watcing his son playing basketball .
他喜欢看他儿子打篮球。