Adverbs modify verbs, adjectives, or other adverbs; they also introduce clauses. There are several categories of adverbs, each with its own rules for adverb word order: conjunctive adverbs, attitude adverbs, sentence adverbs, time adverbs, manner adverbs, and quantity adverbs.
Like adjectives, adverbs can appear in the positive, comparative, or superlative degree.
more rapidly [comparative]
most rapidly [superlative]