Typs of Morphemes

Types of morphemes:
Bound Morphemes: cannot occur on their own, e.g. de- in detoxify, -tion in creation, -s in dogs, cran- in cranberry.
Free Morphemes: can occur as separate words, e.g. car, yes.
In a morphologically complex word -- a word composed of more than one morpheme -- one constituent may be considered as the basic one, the core of the form, with the others treated as being added on. The basic or core morpheme in such cases is referred to as the stem, root, or base, while the add-ons are affixes. Affixes that precede the stem are of course prefixes, while those that follow the stem are suffixes. Thus in rearranged, re- is a prefix, arrange is a stem, and -d is a suffix. Morphemes can also be infixes, which are inserted within another form. English doesn t really have any infixes, except perhaps for certain expletives in expressions like un-effing-believable or Kalama-effing-zoo.
Prefixes and suffixes are almost always bound, but what about the stems? Are they always free? In English, some stems that occur with negative prefixes are not free, such as -kempt.
Morphemes can also be divided into the two categories of content and function morphemes, a distinction that is conceptually distinct from the free-bound distinction but that partially overlaps with it in practice.
The idea behind this distinction is that some morphemes express some general sort of referential or informational content, in a way that is as independent as possible of the grammatical system of a particular language -- while other morphemes are heavily tied to a grammatical function, expressing syntactic relationships between units in a sentence, or obligatorily-marked categories such as number or tense.
Thus (the stems of) nouns, verbs, adjectives are typically content morphemes: "throw," "green," "Kim," and "sand" are all English content morphemes. Content morphemes are also often called open-class morphemes, because they belong to categories that are open to the invention of arbitrary new items. People are always making up or borrowing new morphemes in these categories.: "smurf," "nuke," "byte," "grok."
By contrast, prepositions ("to", "by"), articles ("the", "a"), pronouns ("she", "his"), and conjunctions are typically function morphemes, since they either serve to tie elements together grammatically ("hit by a truck," "Kim and Leslie," "Lee saw his dog"), or express obligatory (in a given language!) morphological features like definiteness ("she found a table" or "she found the table" but not "*she found table"). Function morphemes are also called "closed-class" morphemes, because they belong to categories that are essentially closed to invention or borrowing -- it is very difficult to add a new preposition, article or pronoun.
Try making up a new article. For instance, we would say "Kim bought an apple at the-le fruit stand," meaning "it s the fruit stand (as opposed to anyplace else) where Kim bought an apple;" or "Kim bought an-le apple at the fruit stand," meaning "it s an apple (as opposed to any other kind of fruit) that Kim bought at the fruit stand." This is a perfectly sensible kind of morpheme to have. Millions of West Africans use it every day. However, the chances of persuading the rest of the English-speaking community to adopt it are negligible. In some ways the open/closed terminology is clearer than content/function, since obviously function morphemes also always have some content!
The concept of the morpheme does not directly map onto the units of sound that represent morphemes in speech. To do this, linguists developed the concept of the allomorph. Here is the definition given in a well-known linguistic workbook:
Allomorphs: Non distinctive realizations of a particular morpheme that have the same function and are phonetically similar. For example, the English plural morpheme can appear as [s] as in cats, [z] as in dogs, or [ z] as in churches. Each of these three pronunciations is said to be an allomorph of the same morpheme.