Morphemes can be classified both semantically and structurally. Semantically, morphemes are grouped into general categories: root morphemes and affixational morphemes: simply speaking, roots and affixes. Structurally, they fall into two classes: free morphemes and bound morphemes.
The Root is the most important part of a word that carries the principal meaning. For example, accept in the words acceptable and acceptance is a root. Similarly, -vise in the words vision and supervise is also a root. Affixes are lexically dependent on roots and do not convey the fundamental meaning of words, such as -able in the words speakable and eatable.
Free morphemes can be further divided into two sub-groups: lexical morphemes and functional morphemes. The lexical morphemes include nouns, adjectives, adverbs and verbs which carry the content of the messages we convey. The functional morphemes include conjunctions, prepositions, articles and pronouns. The lexical morphemes form open classes in the sense that their members can be increased with ease. The need for new nouns, adjectives, adverbs and verbs arises frequently in daily life, and additions to these categories occur freely. Functional morphemes are relatively closed because additions to conjunctions, prepositions, articles and pronouns are rare.
It is important to notice that roots are not necessarily free morphemes, and bound morphemes, are not always affixes. The obvious examples are the roots tele- and -vise which are bound morphemes rather than free morphemes. We may diagrammatically describe the relation between roots and free morphemes, and between affixes and bound morphemes in figure 1:
Figure 1: Interrelations between semantic and structural classifications of morphemes
The above figure is interpreted as: all free morphemes are roots, but not all roots are free morphemes. All affixes are bound morphemes, but not all bound morphemes are affixes. Now let us look at some examples.
playground: play(free morpheme, root);
ground (free morpheme, root)
bookstore: book (free morpheme, root);
store (free morpheme, toot);
receive: re- (bound morpheme, affix);
-ceive (bound morpheme, root)
psychology: psycho- ( bound morpheme, root);
-logy (bound morpheme, root)
return: re- (bound morpheme, affix);
turn (free morpheme, root).