同学你好，该知识点来自沪江网校《英语零基础直达PETS三级/公共英语三级【年中特惠班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学好：
The only way to do this was to operate.
the only way 是主语；to do this 是不定式，做定语，修饰way；
was 是系动词；to operate 是不定式，做表语。
这个句子是主系表结构，“搞清楚的唯一方法就是手术”。

The operation, which lasted for over four hours, proved to be very difficult.
The operation proved to be difficult. 是主句。the operation 是主语；proved 是系动词；to be difficult 是表语，to be 是不定式。
which lasted for over four hours 是非限制性定语从句，修饰operation。

祝学习愉快！