什么是副词（4）- 可以修饰动词，形容词，句子，也可以修饰副词本身
https://www.grammarly.com/blog/adverb/
An adverb is a word that modifies (describes) a verb (he sings loudly),
an adjective (very tall), another adverb (ended too quickly),
or even a whole sentence (Fortunately, I had brought an umbrella).
Adverbs often end in –ly, but some (such as fast) look exactly the same as their adjective counterparts.
fast的副词，就是形容词fast本身。