Morphemes can be functionally classified as derivational morpheme and inflectional morpheme. Derivational morpheme is a morpheme that would change the meaning or the part of speech of a word while inflectional morpheme would change the grammar (tense and amount) yet the meaning of the word would not be affected.
In some cases, a morpheme can become both derivational and inflectional. The suffix “-er” for example, is one of the inflectional morphemes that can also inherit the features of a derivational morpheme.
Inflectionally, the suffix “-er” can be added to an adjective so that it becomes a comparative:
smart (adj.) + -er = smarter (comparative)
thin (adj.) + -er = thinner (comparative)
long (adj.) + -er = longer (comparative)
Derivationally, the suffix “-er” is frequently used to generate nouns when it is combined with root verbs:
kill (v.) + -er = killer (n.)
fight (v.) + -er = fighter (n.)
love (v.) + -er = lover (n.)
To summarize, suffixes can be both inflectional and derivational, while prefixes can only be derivational. In terms of flexibility for derivation, suffixes can change a root’s part of speech when two are combined, while new words generated by adding prefixes are bound to the root’s part of speech.