The differences between derivational and inflectional morphology are somewhat ambiguous to explain in some languages. This is also what Bybee (1985: 81) stated in his book, “One of the most persistent undefinables in morphology is the distinction between derivational and inflectional morphology”. It is said so since both deal with morphemes that are usually affixes, either prefixes or suffixes. But in English the distinctions between them are quite recognizable.
Derivational morphology changes the meaning of words by applying derivations. Derivation is the combination of a word stem with a morpheme, which forms a new word, which is often of a different class, for example, develop(V) becomes development(N), developmental(ADJ) or redevelop(V). Thus, derivational morphemes makes new words from old ones (Crystal, p. 90.) as with the suffix of –ion when it is added to the word of create(V) to form another completely separate word of creation(N). Another example, the suffix -ation converts the verb nationalize, into the derived noun nationalization. The suffix -ize converts the noun plural, into the verb pluralize.
Nominalization is a common kind of derivation in English, and it involves forming new nouns from verbs or adjectives, by adding suffixes to them, for example:
Suffix Verb/adjective Derived noun
-ness happy (A) happiness
-ee employ (V) employee
Derivational morphology can be quite complicated, as the classes of words that an affix apply to are not always clearcut, for example the suffix -ee cannot be added to all verbs, ie to add it to run (V) gives runee, which is clearly not an English word.
The characteristics of derivational morphology:
1) Change the part of speech or the basic meaning of a word. Thus -ment added to a verb forms a noun (judge-ment). re-activate means "activate again."
2) Are not required by syntactic relations outside the word. Thus un-kind combines un- and kind into a single new word, but has no particular syntactic connections outside the word -- we can say he is unkind or he is kind or they are unkind or they are kind, depending on what we mean.
3) Are often not productive -- derivational morphemes can be selective about what they'll combine with, and may also have erratic effects on meaning. Thus the suffix -hood occurs with just a few nouns such as brother, neighbor, and knight, but not with most others. e.g., *friendhood, *daughterhood, or *candlehood. Furthermore "brotherhood" can mean "the state or relationship of being brothers," but "neighborhood" cannot mean "the state or relationship of being neighbors."
4) Typically occur between the stem and any inflectional affixes. Thus in governments,-ment, a derivational suffix, precedes -s, an inflectional suffix.
5) In English, may appear either as prefixes or suffixes: pre-arrange, arrange-ment.
While inflectional morphology, in terms of both form and meaning, occupies an unusual position in language, stands between lexicon and syntax in apparent defiance of definition. In most languages inflectional morphology marks relations such as person, number, case, gender, possession, tense, aspect, and mood, serving as an essential grammatical glue holding the relationships of constructions together. Yet in some languages inflectional morphology is minimal or may not exist at all.