Derivational morphology basically involves two general processes, affixation and compounding. There are derivational affixes just as there are inflexional affixes, and many lexemes in many languages are formed by affixation. One of the most important things to understand about derivational morphology, as opposed to inflexional morphology, is that derivational morphology can significantly change the words it plays around with not only in form but in meaning and category. For instance, it is not at all unusual for derivational affixes to change verbs into nouns or adjectives, adjectives into nouns or verbs, that sort of thing. That's what i mean when i say derivational affixation can change category. Essentially, derivational morphology can take a word from any one of the three categories verb, noun, adjective, and return another, related word in either of the other two categories. In  i've given you a bunch of examples of this sort of process taking place in English. In each row of  the word farthest to the left is presumed to be the basic lexeme, or root, to which various derivational affixes are being attached; the derivational affixes are printed in boldface. 
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