The rest of the fish -----by the cat

was eaten.这里的fish明显是鱼肉的意思,所以按不可数名词对待,谓语动词用单数.谓语动词是根据therest of判断 还是根据thefish判断根据the fish. 如果宾语是可数名词复数形式的话，例如“the apples”,你就需要用“were”了。 如"the rest of the apples were eaten by the monkey."