Comparative Adjectives
For most adjectives:

• Form the comparative by adding -ior to the stem for the masculine and feminine form, -ius for the neuter form. The comparative belongs to the 3rd (or i) declension.
• Form the superlative by adding -rimus, -rima, -rimum to adjectives ending in -r; -limus, -lima, -limum to the stems of facilis, difficilis, & similis; and -issimus, -issima, -issimum to the stem of most other adjectives. Superlative adjectives belong to the 1st (a) and 2nd (o) declensions.
• Show comparisons between two things by using quam, with the things being compared in the same case, or by using the ablative case with the second thing being compared (and no preposition).

POSITIVE	COMPARATIVE	SUPERLATIVE
longus, -a, -um	longior, longius	longissimus, -a, -um
miser, misera, miserum	miserior, miserius	miserrimus, -a, -um
acer, acris, acre	acrior, acrius	acerrimus, -a, -um
fortis, forte	fortior, fortius	fortissimus, -a, -um
pulcher, pulchra, pulchrum	pulchrior, pulchrius	pulcherrimus, -a, -um
facilis, facile	facilior, facilius	facillimus, -a, -um

Irregular comparative and superlative degrees:

POSITIVE	COMPARATIVE	SUPERLATIVE
bonus, -a, -um	melior, melius	optimus, -a, -um
magnus, -a, -um	maior, maius	maximus, -a, -um
malus, -a, -um	peior, peius	pessimus, -a, -um
parvus, -a, -um	minor, minus	minimus, -a, -um
multus, -a, -um	--, plus	plurimus, -a, -um
multi, -ae, -a	plures, plura	plurimi, -ae, -a
posterus, -a, -um	posterior, posterius	postremus, -a, -um postumus, -a, -um

Most adjectives ending in a vowel + -us are compared using magis for the comparative and maxime for the superlative. Eg. idoneus suitable, magis idoneus more suitable, maxime ideoneus most suitable. Magis and maxime are adverbs and thus not declined.

Comparative Adverbs

• Adverbs are compared much like adjectives, but adverbs are not declined.
• With regular adverbs, drop the -e or the -iter from the positive form and add -ius for the comparative form. For the superlative form, drop -us/-a/-um from the superlative adjectival form and add -e.

  POSITIVE	COMPARATIVE	SUPERLATIVE
  longe	longius	longissime
  misere	miserius	miserrime
  pulchre	pulchrius	pulcherrime
  acriter	acrius	acerrime
  fortiter	fortius	fortissime
  facile	facilius	facillime

  There are, of course, irregular adverbial comparisons:

  POSITIVE	COMPARATIVE	SUPERLATIVE
  bene	melius	optime
  magnopere	magis	maxime
  male	peius	pessime
  multum	plus	plurimum
  parum	minus	minime
  diu	diutius	diutissime
  saepe	saepius	saepissime