see sb doing sth 是强调看见某人正在做某事,着重动作过程,而see sb do sth 是看见某人做某事,着重于看见这件事的发生.
如：I saw him drawing by the river then.
我见他那时正在河边画画．
I saw him draw by the river sometimes
我看见他有时在河边画画．
－－－by an English teacher