Plan: Structure of English

if, and, live;
driv-er, history-ic, ethnic-al-ly;
act, act-ive, act-iv-ate, re-act-iv-ate.
A morpheme is neither a meaning nor a stretch of sound, but a meaning and a stretch of sound joined together. Morphemes are the smallest indivisible two-facet language units. They are always used as parts of words.
A morpheme that can be a word by itself is called a free morpheme whereas a morpheme that must be attached to another element is said to be a bound morpheme. The morpheme boy, for example, is free, since it can be used as a word on its own; plural -s, on the other hand, is bound. Thus, structurally morphemes fall into free morphemes and bound morphemes. A free morpheme coincides with the stem or a word-form. A bound morpheme occurs only as a constituent part of a word (bound morphemes often signify borrowings). Affixes are bound morphemes, for they always make part of a word.
Morphemes do not always have an invariant form. Morphemes in various texts can have different phonemic shapes. All the representatives of the given morpheme are called allomorphs (from Greek allos "other") of that morpheme. The morpheme used to express indefiniteness in English, for instance, has two forms —a before a word that begins with a consonant and an before a word that begins with a vowel (an orange, an accent, a car). The variant forms of a morpheme are its allomorphs.
Another example of allomorphic variation is found in the pronunciation of the plural morpheme -s in the following words: cats, dogs, judges. Whereas the plural is /s/ in the first case, it is /z/ in the second, and /iz/ in the third. Selection of the proper allomorph is dependent on phonological facts.
Other examples of patterns in which a morpheme's form changes when it combines with another element are easy to find in English. The final segment in assert, for instance, is [t] when this morpheme stands alone as a separate word but [ ] when it combines with the morpheme -ion in the word assertion. Similar alternations are found in words such as permit /permiss-ive, include /inclus-ive, electric /electricity, impress/impress-ion.
Thus, an