Of the eight parts of speech, adverbs tend to be the one people have the hardest time understanding or even remembering. Adverbs probably pose more problems than the other parts of speech, including nouns, pronouns, verbs, adjectives, prepositions, conjunctions and interjections, because adverbs are fluid words, which means different words can function as adverbs depending on their location in a sentence. Adverbs are modifiers and further expound on the meaning of a sentence, and as such can modify several different parts of speech, including verbs, nouns, adjectives, whole phrases or sentences. Also, adverbs can modify other adverbs, so seeing two or more adverbs in a row is not uncommon.
Function of Adverbs
Adverbs are modifiers, which means they change the meaning of verbs, adjectives and even adverbs in the same sentence. For example, in the sentence "John thumbed through the book rapidly," the word "rapidly" provides the reader with more information about how John "thumbed;" thus, "rapidly" is an adverb. Change the adverb to "slowly," and the reader understands "thumbing" differently. In the sentence "John thumbed through the very thick book," the adverb "very" describes the adjective "thick." You could also place the adverb "very" in front of "rapidly" to make an adverb modify another adverb. This results in two consecutive adverbs in a sentence.
Relationship to Other Words in a Sentence
Nouns and adjectives can become adverbs when used to describe other adjectives. For example, you can write "It was an eye-opening experience," with the noun "eye" functioning as an adverb because it modifies "opening." Likewise, in the sentence "The letter-opener drawer is locked," "letter" -- which we typically recognize as a noun -- functions here as an adverb describing "opener." Adverb-adjective combinations usually require a hyphen.
Common Adverb Usage
Sentences like "John thumbed through the book very rapidly" and "She completed the work the least efficiently" contain two consecutive adverbs ("very" and "rapidly," and "least" and "efficiently") and are grammatically correct. In both cases, the first adverb in the sequence modifies the second adverb. You cannot separate the adverbs in these cases and still maintain correct sentence grammar.
Sometimes placing two adverbs in a row sounds awkward. "She really accidentally tripped" or "He runs extremely rapidly" both sound like clumsy mouthfuls. This is because the adverbs all end in "ly." By contrast, the adverb combination in the following sentence flows fine: "Tom is almost always late." The differing endings are therefore easier to pronounce.