Gradable and non-gradable adjectives

Adjectives can be divided into gradable and non-gradable. Gradable adjectives (e.g. difficult, important, happy, tired) are words for qualities that exist in different degrees. Things can be more or less difficult or important; people can be more or less happy or tired. Non-gradable adjectives (e.g. impossible, essential, alive, exhausted) are words for ‘either-or’ qualities. We don’t generally say that some things are more impossible than others, or that somebody is not very exhausted: things are either impossible or not, and people are either exhausted or not.

Degree adverbs like very or more are mostly used with gradable adjectives, and it is mainly gradable adjectives that have comparative and superlative forms.

Note that quite has different meanings in British English with gradable and non-gradable adjectives (see here).