(Please note that this piece simplifies technical details and contains generalisations in aid of brevity.)
The inflectional morphology of a language is the study of the ways in which bound grammatical morphemes combine with stems to be realised as grammatical words. On the other hand, the derivational morphology of a language is the study of the ways in which bound lexical morphemes combine with stems to be realised as lexical words.
Classical grammarians of Latin and Greek generally divided grammar into accidence, word formation and syntax. They did not pay much attention to derivation because they did not really consider it to be a part of grammar.
There are three main differences between inflection and derivation. Firstly, inflection refers to the ways in which bound grammatical morphemes combine with stems to form grammatical words as mentioned earlier while derivation ultimately leads to the formation of lexical words. Both grammatical and lexical words ultimately surface as phonological and orthological words in which bound lexical morphemes can usually be identified as having been affixed. These affixes can be divided into inflectional and derivational affixes. Those which realise bound grammatical morphemes, such as –s, –es on plural nouns, ’s on possessive nouns (though often treated as a clituc) and –ed (or –d after certain stems) which mark both the past tense and the past participle of regular verbs (though irregular verbs such as 'sing', for example, operate quite differently) are called inflectional affixes and have no fixed, concrete meaning of their own (in the sense that despite being meaningful and conveying tense, case or number, they do not create new lexemes) while those which realise bound lexical affixes (such as –ish, –al, –able and –ness) are called derivational affixes.
Classical grammarians of Latin and Greek generally divided grammar into accidence, word formation and syntax. They did not pay much attention to derivation because they did not really consider it to be a part of grammar.
There are three main differences between inflection and derivation. Firstly, inflection refers to the ways in which bound grammatical morphemes combine with stems to form grammatical words as mentioned earlier while derivation ultimately leads to the formation of lexical words. Both grammatical and lexical words ultimately surface as phonological and orthological words in which bound lexical morphemes can usually be identified as having been affixed. These affixes can be divided into inflectional and derivational affixes. Those which realise bound grammatical morphemes, such as –s, –es on plural nouns, ’s on possessive nouns (though often treated as a clituc) and –ed (or –d after certain stems) which mark both the past tense and the past participle of regular verbs (though irregular verbs such as 'sing', for example, operate quite differently) are called inflectional affixes and have no fixed, concrete meaning of their own (in the sense that despite being meaningful and conveying tense, case or number, they do not create new lexemes) while those which realise bound lexical affixes (such as –ish, –al, –able and –ness) are called derivational affixes.
Inflectional affixes do not usually change the grammatical category of the stem although they may enable the stem to perform varying roles: they are usually suffixes which form the outer layer of complex words and do not modify the meaning of the stem in regular ways. This is not the case with derivational affixes which may be either suffixes or prefixes (such as de–, re– and –ize). It is possible for both inflectional and derivational morphemes to occur in the same word. The former generally constitutes the outer layer as affixes tend not to be added after the derivational affix has been added. Thus, inflection may have an input in derivation but derivation does not as often seem to have any input in inflection. For example, in both ‘deindustrialising’ and in ‘depixelating’ the derivational affix ‘de–’ occurs along with a final ‘–ing’ inflectional affix after which no other affix is usually added to either word.
Similarly, if there is both compounding and inflection in a word, the latter must follow the former.
In words in which compounding, derivation and inflection all occur, the inflection is last and compounding is first as can be seen in the words ‘kickstarted’ [(kick + start) + ed] and ‘channelhopping’ [(channel + hop) + ing].
Inflectional morphology not only describes bound grammatical morphemes but also the grammatical rules in which they occur, the paradigm they form and the various orthological and phonological forms in which they eventually surface. Derivational morphology, on the contrary, studies the categories of items with which bound lexical morphemes can be combined, the categories to which the resulting forms belong, the changes in meaning brought on by the process of derivation and the orthological and phonological shapes which bound lexical morphemes acquire.
For the most part, inflectional affixes occur with the members of a given class (though not all of them; irregular verbs and uncountable nouns being exceptions to general rules, for example) unlike derivational affixes which may occur with the members of more than one class or with only some of the members of any particular class. Thus, there are several differences between derivational and inflectional morphology. The most striking though is that the word forms created through the process of inflectional morphology such as ‘talk’, ‘talks’ and ‘talked’ are not new words. They are merely different word forms of the same lexeme. Derivation, however, creates new lexical words with distinct meanings such as ‘amoral’, ‘disown’ and ‘foreground’.