The domain of morphology is words. How words are formed is the concern of this field so morphological structure is the structure which consists of the elements to form words. The most common word formation in language including English is affixation. Affixation is the process of word formation by adding the affixes or bound morphemes in bases or roots (free morphemes). In other words morphological structure is the structure or forms of words primarily through the use of morpheme construct (Crystal, 1980: 232).
Morpheme is defined as the smallest meaningful unit of language (Lim Kiat Boey, 1975 : 37). Morphemes can be divided into two namely free morphemes and bound morphemes. Morphemes are the components which build words. The word singers, for example, consists of three meaningful units or morphemes, sing, –er, and –s. The morpheme sing which forms the word singers has the lexical meaning; the morpheme –er means the doer of singing; the morpheme –s has plural meaning. We can identify the meaning of the morpheme sing although it stands alone but we cannot identify the meaning of morphemes –er and –s in isolation. We can identify the meaning of the morpheme –er and –s after they combine to the morpheme sing. Sing which can meaningfully stand alone is called free morpheme while the morphemes such as –er and –s, which cannot meaningfully stand alone are called bound morphemes. Bound morphemes must be attached to free morphemes. Bound morphemes are also called affixes which can be classified into prefix, infix, and suffix. English only has two kinds of bound morphemes namely prefixes and suffixes. No infixes exist in English. Bound morphemes are classified into two types namely derivational and inflectional morphemes. Both inflectional and derivational morphemes play an important role in the larger structure namely syntactic structure.
