Many English adjectives exhibit three forms, for example GREEN here:

(30) Grass is green.

(31) The grass is greener now than in winter.

(32) The grass is greenest in early summer.

The grammatical words that green, greener and greenest express are the positive, comparative and superlative of GREEN, contrasting on the dimension of comparison. Other adjectives with similar forms are:

(33)    Positive             Comparative                 Superlative

           happy                 happier                      happiest

           long                    longer                        longest

           pure                    purer                         purest

          untidy                  untidier                      untidiest

           good                   better                         best

All these exhibit a regular pattern of suffixation with -er and -est, except for better and best, which are suppletive.

The justification for saying that comparative and superlative forms of adjectives belong to inflectional rather than to derivational morphology is that there are some grammatical contexts in which comparative or superlative adjectives are unavoidable, anything else (even if semantically appropriate) being ill-formed:

(34) a. This field is greener than that one.

        b. *This field is green than that one.

        c. *This field is fertile than that one.

(35) a. The greenest fields of all are here.

        b. *The green fields of all are here.

        c. *The superior fields of all are here.

On the basis of our experience with plurals of countable nouns and past tense forms of verbs, then, you will probably expect that every adjective lexeme should possess a comparative and a superlative form (or, at any rate, every adjective denoting a property that can be present to a greater or lesser degree). However, it is striking that many adjectives lack these forms:

(36) *Curiouser and curiouser!

(37) *This field is fertiler than that one.

(38) *The fertilest fields of all are here.

(You may recognize (36) from Lewis Carrol’s Alice’s Adventures in Wonderland as something that Alice scolds herself for saying.) But it is not that the content of (36)–(38) is inexpressible in English; rather, instead of the suffixes -er and -est, we use periphrastic forms with more or most:

(39) More and more curious!

(40) This field is more fertile than that one.

(41) The most fertile fields of all are here.

Broadly speaking, the suffixes -er and -est appear on adjectives whose basic form has one syllable, or two provided that the second syllable ends in a vowel (e.g. tidy, yellow), while longer adjectives usually require the periphrasis.