Adjectives can be divided into two different types: gradable adjectives which have superlative and comparative forms (e.g. big, bigger, the biggest) and ungradable adjectives which have no such forms (e.g. British, metal, huge).
Head to the Random Idea English Blogspot for fiendishly difficult exercises on this theme.

Gradable V Ungradable (exercises)

Gradable V Ungradable (table)