其实有时两者没有区别、有时两者有些区别,最简单的方法是改写.例如：
I saw him walk into the room.可以改成He walked into the room.
I saw him walking into the room.改成He was walking into the room.
改写后可以发现这两句差别不大,只是你在说话时侧重点在全过程用do、侧重点在正在发生的过程用doing,两个都对.但如果要说“我看到他走进屋子还锁了门”就不能用ing形式：
I saw him walk into the room and lock the door.
因为必须待walk动作全过程完成后才能做lock的动作.
有just now最好用walking.