Free morphemes are those which may occur alone or constitute words by themselves. All mono-morphemes are free morphemes and poly-morphemic words, which consist wholly of free morphemes, are compounds. For example, “dog”，“nation” and “close” are free morphemes. Free morphemes can be divided tato two categories. The first one is the set of ordinary nouns, verbs and adjectives which carry the content of messages we convey. These free morphemes arc called lexical morphemes and, since we can create new lexical morphemes for the language rather easily, they are called an open class of wards. The second category of free morphemes is called functional morphemes. As we almost never add new functional morphemes to the language, they are called a closed class of words.