Short answers, pulled from the story.
An adverb is a word or expression that modifies a verb, adjective, another adverb, a determiner, a clause, a preposition, or a sentence. Adverbs typically express manner, place, time, frequency, degree, or level of certainty. The function can be performed by a single adverb, an adverbial phrase, or an adverbial clause.
The word adverb derives through French from the Latin adverbium, built from ad- meaning "to," verbum meaning "word" or "verb," and the nominal suffix -ium. The etymology reflects the original idea that adverbs principally attach themselves to verbs.
Modern grammarians describe the adverb as a catch-all category because it groups together words with very different syntactic behaviors, united mainly by the fact that they do not fit into noun, adjective, or preposition classes. Words like very, here, not, and certainly are all labeled adverbs despite operating in entirely different grammatical slots.
In English, adverbs of manner are often formed by adding -ly to an adjective, but flat adverbs such as fast, slow, and friendly share the same form as the corresponding adjective. Comparison is usually expressed with more and most before the adverb, though a few adverbs like well use inflected forms: better and best.
Portuguese, Spanish, Galician, and Italian form many adverbs by adding -mente to the adjective, typically the feminine form. French and Catalan use -ment. Both suffixes descend from the Latin mens, mentis, meaning mind or intelligence, or the suffix -mentum meaning result or way of action. Romanian is an exception: almost all its adverbs are simply the masculine singular form of the corresponding adjective.
Not has been traditionally classified as an adverb, but grammarians note it does not behave grammatically like any other adverb in English. It cannot modify adjectives as very does, cannot head a sentence as certainly can, and targets the truth value of a proposition in a unique way. The prevailing scholarly view is that not probably should be placed in a class of its own.