They often watch fish ___ in the fish tank.(swim)

问题描述:

They often watch fish ___ in the fish tank.(swim)

答案是:swim
watch是一个感官动词,后面接宾语补足语的时候可以有两种情况.一是接动词原型,二是接动词ing形式,但意思却强调观察到正在做某事.