The parts of speech explain how a word is used in a sentence.There are eight main parts of speech (also known as word classes): nouns pronouns adjectives verbs adverbs prepositions conjunctions and interjections.Most parts of speech can be divided into sub-classes. Prepositions can be divided into prepositions of time prepositions of place etc. Nouns can be divided into proper nouns common nouns concrete nouns etc.