语法上分析的不定式做结果状语，并非都是表示逻辑上的因果关系。

too... to do, 和 so... as to do, 可以理解表示因果关系。例如：

He is too weak to speak a word. = He is so weak that he can't speak a word.

He is so careless as to make many spelling mistakes. = He is so careless that he has made many spelling mistakes.

有时不定式作结果状语仅仅表示后来发生的事情。例如：

She lived to be 110 years old. 她货到了110岁。语法书把这个不定式分析为结果状语。你也可以理解不定式为程度状语。

She lived to see her granddaughter marry the prince. 她活到了看到孙女嫁给王子的那一天。语法书把这个不定式分析为结果状语。

不定式还常用来表示预料之外的不好的结果。例如：

He rushed home, only to find his father had died. 本来他赶回家要送父亲去医院，不料却发现父亲已经去世了。这种only to do表示预料之外的情况，语法书分析为结果状语。

He drove as fast as he could to the station, only to be told the train had left.

你可能会问，既然不是因果关系，为什么要分析为结果状语呢？问得好。但你觉得应该分析为什么状语？这里模糊一下，理解句子的意义其实已经足够了。