Comparative and Superlative

For comparisons of equality, use the construction so + adjective or adverb + wie to mean as + adjective or adverb + as.  You can also add nicht in front of the so for a comparison of inequality.

Die Küche ist so gross wie das Wohnzimmer.  The kitchen is as big as the living room.
Eine Waschmaschine ist nicht so schwer wie ein Kühlschrank.  A washing machine is not as heavy as a refrigerator.

Comparative

1.  For comparisons of superiority and inferiority, add -er to the adjective or adverb, followed by als (than).  German always uses the -er ending, although English sometimes uses the word more before the adjective instead of the ending.

Ein radio is billiger als ein Fernseher.  A radio is cheaper than a TV.
Jens läuft schneller als Ernst.  Jens runs faster than Ernst.
Lydia ist intelligenter als ihr Bruder.  Lydia is more intelligent than her brother.

2.  Adjectives that end in -el, -en or -er, drop the -e in the comparative form.  Teuer becomes teurer instead of teuerer, and dunkel becomes dunkler instead of dunkeler.  Some one-syllable adjectives and adverbs whose stem vowel is a, o, or u add an umlaut in the comparative, such as alt, arm, dumm, grob, groß, hart, jung, kalt, klug, krank, kurz, lang, oft, scharf, schwach, stark, warm.  Adjectives that never add an umlaut are flach, froh, klar, rasch, roh, schlank, stolz, toll, voll and zart.

Superlative

1.  To form the superlative, add -(e)st to the adjective.  The ending -est is used when the word ends in -d, -t, or an s sound.  The adjectives that end in -el, -en, or -er retain the -s in the superlative form.  The same adjectives that took an umlaut in the comparative take an umlaut in the superlative as well.

2.  The superlative also has an alternative form:  am + adjective or adverb + sten.  When the adjective or adverb ends in a d, t or s sound, an e is inserted between the stem and ending (am grössten is an exception.)  This is the only form of the superlative of adverbs, but either forms of the superlative can be used for adjectives.

Hans is am jüngsten.  Hans is the youngest.
Sie ist am intelligentesten.  She is the most intelligent.

Irregular Forms

Common forms of the comparative

Je mehr, desto besser.  The more, the better.
Je mehr Geld er hat, desto glücklicher ist er.  The more money he has, the happier he is.
Die preise werden immer höher.  The prices are getting higher and higher.
Julia wird immer hübscher.  Julia is getting prettier and prettier.

Keep in mind that the comparative and superlative forms take normal adjective endings when they precede a noun.  And the adjective form of the superlative must always take an adjective ending because it is preceded by the definite article.

Haben Sie billigere Anzüge?  Do you have less expensive suits?
Diese Anzüge sind die billigsten.  These suits are the least expensive.