The words of every language are divided into several word classes, or parts of speech, such as nouns, verbs and adjectives etc. The words of a given class exhibit two or more forms in somewhat different grammatical circumstances. These forms are not interchangeable and each can be used only in a given grammatical situation. This variation in form is required by the existence of a grammatical category applying to that class of words. Thus a grammatical category is "a linguistic category which has the effect of modifying the forms of some class of words in a language" (Trask). For example, English nouns have the grammatical category of number. Thus the singular `dog' and the plural `dogs' exist but are not interchangeable in a sentence. A noun can be used only in its singular or plural form as there is no possibility of another form. English adjectives vary for...