The evening of this day was very long and melancholy, at Hartfield. Theweather added what it could of gloom.
原文源于《傲慢与偏见》。
could后面要接省略to的动词不定式，但这里除去could外没其他的动词了，那样这里的what是作什么成分？是代替省略to的不定式 be吗？
The weather added much of gloom. 这个句子非常了解，much为代词作added的宾语，of gloom是much的定语。作为代词的much也是可以有比较级的，所以much可以扩展为 as much as it could (could后省略了add）, 得到 The weather added as much as it could of gloom. added的宾语仍然是as much as it could，of gloom为其定语。然后用what it could（could后省略了add）替换as much as it could, 则what it could这个名词性关系分句作added的宾语，what= the amount that, of gloom是what中隐含的the amount的定语。
所以，The weather added what it could of gloom.= The weather added much of gloom.
