The function or role a word plays in the sentence is probably the most important aspect in our understanding of English grammar. For instance, we may normally think of the word telephone as a noun, a part of speech that names a person, place, concept, or thing.
The telephone is in the kitchen.
In the above sentence, telephone, a noun, functions as the simple subject.
However, in the following sentence telephone becomes an adjective, which is a modifier, because it describes the word table, itself a noun and the subject of the sentence. Anything that describes or modifies a noun is an adjective, a modifier.
The telephone table is in the kitchen.
In the sentence below, the same word telephone becomes a finite verb--a word that clarifies some sense of time in which the action occurs (here present tense) and that expresses an action--because it asserts what the subject They does.
They telephone their daughter every Sunday morning.
The ability of the same word to play different roles--or functions--in various sentences adds to the flexibility of the language. It is best, therefore, not to think of any one word as always being a noun, adjective, or verb, or as always being a subject, modifier, or finite verb. Rather, we should look at how the word is used in each sentence in which it appears. We simply ask ourselves if the word names, describes, asserts or does something else in the sentence. Based on the answer to our question, we can then say what part of speech the word is and what function (subject, finite verb, direct object, etc.) it plays in that particular sentence.
