Free Morphemes: Content Words and Function Words
The irreducible core of any word is its root – it is a non-affix morpheme that cannot be analyzed into even smaller elements. For example, talk is a root, which pops up in a wide range of word-forms that instantiate the lexeme TALK as, for instance, in talking, talked, talks or talker or the root write and the word-forms writer, written, writes which are all realizations of the same lexeme, namely WRITE. Those roots, which are capable of standing independently and in complete isolation without any attachment, are referred to as free morphemes.
It is useful, however, to subdivide the group of free morphemes into two different classes of words: lexical content words and function words.
The former category of lexical content words comprises nouns, verbs, prepositions, adjectives, and adverbs, words that by and large carry an intrinsic meaning and thus refer to an extralinguistic reality.
Lexical content words represent an open class since new words can easily be added to this class. Consider for instance word creations such as nerd, to google, to bogart or ADS and PC that found their way into the lexicon.
Function words belong to a closed class. A closed class is restrictive in the sense that no new words can be added to this class. While lexical morphemes carry most of the semantic meaning, function words have a structural function – function words primarily though not exclusively indicate grammatical relations. Articles (the and a), pronouns (I, you, they, we, them, your, hers etc.), conjunctions (e.g. but, if, yet, and) and demonstratives (these, those, that, this) are instances of function words.