EXPLANATION AND EXERCISES:
http://www.ego4u.com/en/cram-up/grammar/adjectives-adverbs/adjectives
MORE EXERCISES:
http://www.saberingles.com.ar/curso/lesson12/05.html
http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/regcom1.htm
http://www.usingenglish.com/quizzes/45.html
http://www.ego4u.com/en/cram-up/grammar/adjectives-adverbs/adjectives/exercises
http://www.ego4u.com/en/cram-up/grammar/adjectives-adverbs/adjectives/exercises?02
http://www.ego4u.com/en/read-on/countries/usa/tour/los-angeles#exercises
http://www.ego4u.com/en/read-on/countries/uk/tour/london#exercises
http://www.englisch-hilfen.de/en/exercises/adjectives_adverbs/adjective_comparison.htm
http://www.englisch-hilfen.de/en/exercises/adjectives_adverbs/adjective_comparison2.htm
http://www.englisch-hilfen.de/en/exercises/adjectives_adverbs/adjectives_comparison_sentences2.htm
http://www.englisch-hilfen.de/en/exercises/adjectives_adverbs/adjectives_comparison_as_as.htm
http://www.englisch-hilfen.de/en/exercises/adjectives_adverbs/adjective_comparison4.htm
http://www.englisch-hilfen.de/en/exercises/adjectives_adverbs/adjective_comparison5.htm