To linguists, inflectional morphology is the study of how inflections, or changes to a word’s most basic form, changes meaning. To the rest of us, it’s a matter of plurals, tense, and just plain good sense. Whether a word gains or loses a prefix or suffix, undergoes an internal vowel or consonant change in order to become plural, or is otherwise transformed is the stuff of inflectional morphology.
A morpheme might sound like what certain types of children’s toys do when they are transformed from a kitten to a space beast with the twist of a screw, but in fact, it’s a unit of sound. To be precise, a morpheme is the smallest meaningful unit of sound in English or in any other language. Some morphemes are words of a single syllable, for example, /bird/. While adding an s to the word doesn’t change its single status, it does add another morpheme because /s/ has a meaning, as well; it means "more than one."
Words in English don’t just stand around being their most obvious selves. Words that are nouns in one sentence can suddenly leap onto tiptoe and dance as a verb in the next. Screwing on a suffix can change a word into an adverb, and adding a prefix to a noun or even to a verb can give it a whole different universe of meaning. If this were not the case, English would need a whole lot more than the roughly 175,000 functional words that compose it.
Prefixes such as "pre," "sub," and "re" change words in ways that are unpredictable as well as in ways that make sense. If a bunch of people clap when a speaker is finished, they make a sound. On the other hand, if the room resounds with applause, it isn’t sounding again, it’s spilling over with more sound that it seems able to contain. A scribe is trained to write well, but when the doctor prescribes, he or she is not working with people who are in school to become scribes but aren’t yet graduated.
Suffixes like "ly," "ed," and "ment" also change meaning via inflectional morphology. Adding the morpheme /ly/ to an adjective, for example, changes it into an adverb: She is quick because she runs quickly. While the morphemes /ed/ and /s/ can differentiate between past and present or how many people are doing something, adding the morpheme /ment/ turns an adjective into a noun, as in "contentment."
Inflectional morphology isn’t just about turning words into other words with prefixes and suffixes. It can also involve internal vowel changes that affect meaning. “I sing every day” means that I sing today, I sang yesterday, and I will sing tomorrow; I always have and always will. “I sang every day” means that, in the past, there was a time period during which I sang each and every day but that I no longer do. The change of a single letter on paper or sound in the linguistic stream makes an enormous difference in what is meant.