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 degree; verbs for tense; pronouns for case etc.
Traditional grammarians divide the words of English into eight classes or parts of speech- noun, pronoun, adjective, verb, preposition, conjunction, adverb and interjection. The words of a particular class can have two or more forms in different grammatical circumstances because of the existence of a grammatical category. Grammatical categories are an essential part of traditional grammar especially of the classical languages. As English is modelled on the grammar of Latin, it follows the system of Latin. The most common grammatical categories in English are gender, number, person, tense, mood, voice and case.
The grammatical category applying to the English nouns is number. Therefore we have the singular `dog' and the plural dogs'. Most of the nouns in the English follow the same pattern. These forms are not interchangeable. Each of these forms can be used only in the specified grammatical circumstance. A noun can be used only in its singular or plural form. There is no other form which can be used, which is not marked one way or the other. All grammatical categories share this feature. Therefore grammatical categories cause the variation in the form of the English nouns in different grammatical circumstances. English nouns do not have the category of case but nouns in Latin, German, Russian and many other languages are varied for case.
As the English nouns are varied for number, the English pronouns vary for case. In a given position only one form can exist. Examples are I/Me, she/her and they/them. English adjectives have the grammatical category of degree. Thus big/bigger/biggest are the three forms of the adjective big and only one these forms can occupy a given position in a grammatical circumstance. English verbs have the category of tense. Examples are love/loved, work/worked, see/saw etc. Some of these verbs are irregular but only one of them occurs in a given position. The difference between the two forms is illustrated in the following sentences:
I see her. (Now)
I saw her. (In the past)
In the first...