Adverbs are a type of word whose main function is to complement the verb. However, it is not the only one, because in the same way an adverb can act as a complement to an adjective and also as a complement to another adverb.
Actually, this complementary work is very, very similar to what an adjective does with a noun. In fact, it is so similar that some linguists have called the adverb "the adjective of the verb." Adverb comes from the Latin “ad-verbum”, which literally means “next to the verb”. That is why its main function, as we have said, is to “adjectivize” the verb.
As it happens with nouns and adjectives, adverbs have the ability to radically change the way in which the verbal action is manifested, or at least how we perceive it. We can say that “the boy ran”, but we will vary the sentence a lot if we say that “the boy ran slowly” or “the boy ran fast”. We can say that "our friend arrived", but that will be very different from saying that "our friend arrived late". Very different.
Adverbs, then, are fundamental pieces when dressing verbs, presenting them with a certain complexity and depth.
But we have said that adverbs can also complement adjectives and other adverbs. Let's see some example.
I think your friend is pretty slow
Here, “quite” is modifying to“slow”, an adjective (we could say “very slow”, “somewhat slow”, “not slow at all”, etc.). And the function, if we look closely, is almost identical to that of adjectives.
When he warned me it was already too late
Here, “too much” is complementing (or adjectivizing) “late”, an adverb. The fact that both words are adverbs is not an apex so that the function that too much performs here is not exactly identical to the one that “enough” performed before, in the previous example.
Among the latter, that is, among the adverbs that modify adjectives and other adverbs, some have become so specialized that they have ended up adopting a shortened form. This is the case of the adverbs “muy” (of much), “cuán” (of how much), and “tan” (of so much), which will never appear as complements of a verb, Another type of very common adverbs are those that end in “-mente”. Almost any adjective can be adverbialized in this way, by adding “-mente” to its feminine form (good » well, bad » badly, sad » sadly, effective » effectively).
On the other hand, there are many types of adverbs, among which the adverbs of affirmation, negation and doubt stand out, those of time, place and frequency, as well as those of manner (category to which most of those ending in “-mente”.