Formation of degrees of comparison

The comparative is formed with the positive form of the adjective and the comparative suffix -er ;  the superlative is formed with the positive form of the adjective and the superlative suffix -st.

Many monosyllabic adjectives with the vowels a, o, u in their stem take an umlaut in the comparative and the superlative form.

Adjectives ending in –el: Adjectives ending in –el lose the unstressed e of the ending in the comparative, but not in the superlative.

Adjectives ending in –er: If the stem of an adjective ends in a diphtong + er , the e of the final syllable is omitted in the comparative. This rule also applies to foreign origin words that end in an unstressed -er.

All other German adjectives ending in an unstressed  -er oder -en can  lose the e of the stem in the comparative (not obligatory).

The unstressed –e of an adjective ending is omitted during the formation of the degrees of comparison.
 

Monosyllabic and polysyllabic adjectives stressed in the final syllable and ending in d, t, s, sch, ß, sk, x, or z, form the superlative with –est instead of –st. 

Exceptions to this rule are the adjective groß, adjectives ending in –isch, adjectives that derive from the present participle or those ending in –ert, -elt or –tet and derive from the past participle.

Certain adjectives form the degrees of comparison irregularly.