The German comparative is formed with the ending -er. Actually it is identical with the English form of single-syllable or two-syllable words (fast => faster; happy => happier). If you suspect that it cannot be so easy, you are right. However, exceptions are only a few as we will see soon. Concerning the formation of comparatives there is no difference between adjectives and adverbs. Since quite often the same word can be used as adjective or as adverb, this is quite easy to understand. In English there is the ending -ly distinguishs the adverb from the adjective (The car is slow. [Das Auto is langsam.] / She walks slowly [Sie läuft langsam].) All the following rules apply to adjectives as well as to adverbs.