1998witnessed sth happening ----
Do you witness illegal acts happening in your neighborhood?
确切的说,witnessed 后面的宾语补足语可以使doing也可以是done.具体取决于宾语和宾补之间的主被动关系.本题中sth 与happen 之间是主动关系,因此用happening的形式.
如果是被动,则用done的形式.如:witness Tom hit by his classmate.witness the water polluted by people.