Charles Carpenter Fries (1964) classified the words into two categories; they are class words and function words.
1. Class words
Noun is one of the most important parts of speech. Its arrangement with verb helps to form the sentence core which is essential to every complete sentence. Nouns are divided into several kinds, they are:
* Proper nouns are the words begin with capital letter in writing.
o Personal names: Mr. John smith
o Names of countries, cities, rivers: Holland, Paris
o Names of holidays: Thanksgiving
o Names of time units: Saturday, June
o Words used for personification-a thing or abstraction treated as a person: Nature, Liberty.
* Concrete or abstract nouns. Concrete noun is a word for physical object that can be perceived by the sense: flower, girl. Abstract noun is a word for concept-it is an idea that exists in our mind only: beauty, justice, mankind, etc.
* Countable or uncountable nouns. Countable noun consists of singular and plural noun: one girl, two girls. Uncountable noun: coffee, sugar, water.
* Collective nouns are group of people, animals or objects considered as a singe unit: audience, class, faculty, etc.
* Noun compounds refer to a group of words
Noun + noun: department store
Adjective + noun: black bird
Verb + noun: pickpocket
Verb is the most complex part of speech. Its varying arrangements with nouns determine the different kinds of sentences- statements, questions, commands, or exclamations. It consists of transitive and intransitive verbs. Transitive verb take direct object (He is reading book). Intransitive verbs do not required direct object or may be used in the passive voice (He is walking in the park)
Adjective is a modifier that has grammatical property of comparison. For example: beautiful, old, big, hot, etc.
Adverbs range in meaning from words having a strong lexical content that describe the action of the verb, or those that indicate such meanings as time and place. It consists of adverbs of manner (quickly, neatly, hardly); place adverbs (here, away, outside); time adverb- definite time (today, tomorrow) and indefinite time (recently, soon, immediately); adverbs of frequency (often, sometime, always).
2. Function words
Function words can be subdivided into 6 groups as follow:
a. Determiners (the, an, a)
Determiners are the function words which always occur with nouns to form noun phrase. For example: the student, a table, an apple, etc.
b. Auxiliaries ( am, is, was, were, can, may, must, should, etc)
Auxiliaries are the function words which always occur with verbs to form verb phrases. For example: I am reading book.
c. Intensifiers (very, so, too, quiet, etc)
Intensifiers are the function words that always occur with adjectives or adverbs to form adjective phrases or adverb phrases. They function as modifiers of adjectives or adverbs. For example: She is very beautiful.
d. Prepositions (on, in, under, above, etc)
Prepositions are the function words that can occupy the position of the word. For example: The newspaper is on the table.
e. Conjunctions (and, so, but, so that, when, etc)
Conjunctions are function words that can occupy the position of the word. It is the member of a small class that has no characteristic form. For example: I study English and you study mathematics.
f. Question Words (who, what, where, when, why, how)
Question words are function words used as signals of question sentences. For example: What is your name?