Morphological Processes: Derivation versus Inflection
As already mentioned in previous sections, affixes are bound morphemes. However, affixes can be further categorized into two kinds: inflectional and derivational.
Although the distinction between derivation and inflection is widely accepted within the field of morphology, it still remains one of the most controversial issues in morphological theory.
For now, however, it may suffice to first delineate the most striking differences between these two word building processes:
Inflectional affixes produce a new word from of an existing lexeme a word i.e. they do not create a new entry in one's mental lexicon. For example, the noun "students" can be produced by adding the plural -s, a inflectional suffix, to the base "student". The plural -s indicates that more than one student is concerned, but it does neither change the grammatical category of the word nor does it produce a new lexeme. Additionally to number, inflectional affixes give grammatical information in terms of tense, case and gender.
Derivational affixes, in contrast, are capable of creating a new lexeme from a base. Therefore, they can provide a more complex change. On the one hand, a derivational morpheme can change the grammatical category of the word. A derivational suffix like "-ly" can transform an adjective into an adverb, the suffix "-ment" is often used to produce a noun. On the other hand, we can change the meaning of a word without changing its category. If we add the derivational prefix "un-" to the adjective "happy", we receive the adjective "unhappy". The word remains an adjective while the meaning changes completely.
There are several factors which indicate whether an affix is derivational or inflectional. For one, it is essential to keep in mind that any prefixes in the English language are derivational. In her book on Morphology from 1988, Laurie Bauer summarizes the factors which can help you to differentiate between inflectional and derivational affixes:
"(a) If an affix changes the part of speech of the base, it is derivational. Affixes which do not change the part of speech of the base are usually (though not invariably) inflectional. So form is a noun, formal is an adjective; -al has changed the part of speech; it is thus a derviational affix. Formal is an adjective, formalise is a verb; -ise has changed the part of speech; it is a derviational suffix. Formalise is a verb, formalises is still a verb; -'s' has not changed the part of speech; -'s' is likely to be an inflection affix. Note, however, that while all prefixes in English are derivational, very few of them change the part of speech of the base.
(b) Inflectional affixes always have a regular meaning. Derivational affixes may have irregular meaning. If we consider an inflectional affix like the plural 's in word-forms like bicycles, dogs, shoes, tins, trees, and so on, the difference in meaning between the base and the affixed form is always the same: 'more than one'. If, however, we consider the change in meaning caused by a derivational affix like 'age in words like bandage, cleavage, [...] peerage, shortage, spillage, and so on, it is difficult to sort of any fixed change in meaning, or even a small set of meaning changes.
(c) As a general rule, if you can add an inflectional affix to one member of a class, you can add it to all members of the class, while with a derviational affix, it is not generally possible to add it to all members. That is, inflectional affixes are fully productive, while derivational affixes are not. For example, you can add -s to any non-modal verb in English to make the 'third person singular of the present indicative', but you cannot add -ation to any non-modal verb to make a noun. [...] We can summarise this criterion in the following way: affixes which show limited productivity with large numbers of gaps are derivational; affixes which are fully productive can be either inflectional or derivational." (Bauer 1988, 12f.)