帮忙分析一下这个英文句子的结构

题目

帮忙分析一下这个英文句子的结构
Only in the most exceptional circumstances is the lifespan of a patent extended to alter this normal process of events.

答案

（Only in the most exceptional circumstances 状语）（is系动词）（ the lifespan of a patent 主语）（extended to alter this normal process of events表语）only放句首引导状语,后边的句子半倒装
正常语序：(the lifespan of a patent )is extended(被动） to alter this normal process of events only in the most exceptional circumstances