Adverbs modify, or describe, other words such as verbs, adjectives, and other adverbs.
Some adverbs, though, are used for making comparisons and are called comparative and superlative adverbs. If more and most are not used, the dictionary will list the appropriate suffix. Decide how many things you are comparing. If you are comparing only two objects, use the comparative forms, -er or more. My Escort moves more slowly than Ashleigh's Porsche.