An adverb is a word or group of words that modifies or describes a verb. Many one-word adverbs end in “-ly,” such as he ran quickly. Others, however, do not, such as he ran fast.
Adverbs also modify or describe adjectives, such as the very tall man. They may also modify other adverbs, such as he ran extremely fast.
Adverbs typically serve to answer what we might call adverbial questions about the verbs they modify. These include:
Under what circumstances
Adverbs come in three states. In the positive state, the adverb ascribes the basic attribute, as in He wrote clearly. To form the comparative state, we usually add the word more, as in He wrote more clearly. To form the superlative state, we usually add the word most, as in He wrote most clearly. These rules apply to the “-ly” adverbs.
Adverbs not ending in “-ly” can form their comparative state by adding “-er” (he ran faster) and their superlative state by adding ‑est (he ran fastest).
You'll find a complete discussion of this problem in the section on Adverbs in Parts of Speech on Grammar.com. Click here for the beginning of that discussion.
See also compound verb about the placement of adverbs in multiword verb forms, such as We have definitely decided to have a party.