In English morphology, an inflectional morpheme is a suffix that's added to a word (a noun, verb, adjective or an adverb) to assign a particular grammatical property to that word, such as its tense, number, possession, or comparison. Inflectional morphemes in English include the bound morphemes -s (or -es); 's (or s'); -ed; -en; -er; -est; and -ing. These suffixes may even do double- or triple-duty. For example, - s can note possession (in conjunction with an apostrophe in the proper place), can make count nouns plural, or can put a verb in the third-person singular tense. The suffix -ed can make past participles or past-tense verbs.
Kristin Denham and Anne Lobeck, authors of "Linguistics for Everyone," explain why there's overlap: "This lack of distinction in form dates back to the Middle English period (1100-1500 CE), when the more complex inflectional affixes found in Old English were slowly dropping out of the language."
Contrast With Derivational Morphemes
Unlike derivational morphemes, inflectional morphemes do not change the essential meaning or the grammatical category of a word. Adjectives stay adjectives, nouns remain nouns, and verbs stay verbs. For example, if you add an -s to the noun carrot to show plurality, carrot remains a noun. If you add -ed to the verb walk to show past tense, walked is still a verb.
George Yule explains it this way:
"The difference between derivational and inflectional morphemes is worth emphasizing. An inflectional morpheme never changes the grammatical category of a word. For example, both old and older are adjectives. The -er inflection here (from Old English -ra) simply creates a different version of the adjective. However, a derivational morpheme can change the grammatical category of a word. The verb teach becomes the noun teacher if we add the derivational morpheme -er (from Old English -ere). So, the suffix -er in modern English can be an inflectional morpheme as part of an adjective and also a distinct derivational morpheme as part of a noun. Just because they look the same (-er) doesn't mean they do the same kind of work." ("The Study of Language," 3rd ed. Cambridge University Press, 2006)
When building words with multiple suffixes, there are rules in English that govern which order they go in. In this example, the suffix is making a word into a comparative:
"Whenever there is a derivational suffix and an inflectional suffix attached to the same word, they always appear in that order. First the derivational (-er) is attached to teach, then the inflectional (-s) is added to produce teachers." (George Yule, "The Study of Language," 3rd ed. Cambridge University Press, 2006)
"Linguistics for Everyone" lists additional examples to drive home the point about placement order of the affixes: "For example, the words antidisestablishmentarianism and uncompartmentalize each contain a number of derivational affixes, and any inflectional affixes must occur at the end: antidisestablishmentarianisms and uncompartmentalized." (Kristin Denham and Anne Lobeck. Wadsworth, 2010)
The study of this process of forming words is called inflectional morphology.