英语语法,下面的句子怎样分清主谓宾语

题目

英语语法,下面的句子怎样分清主谓宾语
Screws which are operated for maintenance purposes shall not be of insulating material if their
replacement by a metal screw could impair supplementary or reinforced insulation.

答案

Screws（主语）shall not be （谓语）of insulating material （介词结构做表语,此句无宾语）.which are operated for maintenance purposes （定于从句,修饰螺丝）if theirreplacement by a metal screw could impa...