A word is a “part of speech” only when it is used in a sentence. The function the word serves in a sentence is what makes it whatever part of speech it is.
For example, the word “run” can be used as more than one part of speech:
Sammy hit a home run. (run is a noun, direct object of hit)
You mustn’t run near the swimming pool. (run is a verb, part of the verb phrase must (not) run)
Here is a simple overview of the English parts of speech and what they do. Each part of speech is linked to an DWT article that tells more about it.
NOUN – Nouns are naming words. We can’t talk about anything until we have given it a name.
PRONOUN – A pronoun is a word that stands for a noun.
VERBS – The verb is the motor that runs the sentence. A verb enables us to say something about a noun.
ADJECTIVE – An adjective is a word that describes a noun.
ADVERB – An adverb adds meaning to a verb, an adjective, or another adverb.
PREPOSITION – a preposition is a word that comes in front of a noun or a pronoun and shows a connection between that noun or pronoun and some other word in the sentence
CONJUNCTION – a conjunction joins words and groups of words.
INTERJECTION — An interjection is a word or phrase thrown into a sentence to express an emotion, for example, Homer Simpson’s “Doh!”