Parts of speech

Parts of speech are types of word in grammar. There are many different word : they are called 'lexical categories'. The most common are these:
Open and closed classes.
Verbs, nouns, adjectives, adverbs, and interjections are called open classes because they are parts of speech that can easily add new words. On the other hand, pronouns, prepositions, and conjunctions are closed classes because new words cannot be added easily. For example, since pronouns are a closed class, there are fairly few pronouns: "I, me, my, mine, myself, you, your, yours, yourself, he, him, his, his, himself, she, her, her, hers, herself, it, it, its, its, itself, we, us, our, ours, ourselves, they, them, their, theirs, themselves". Since pronouns are used to replace whole noun phrases, there is no need to have many kinds of pronouns. Instead of saying "The Earl of Sandwich introduced the Earl of Sandwich's favorite food, the sandwich", one uses the pronoun "his" to replace "the Earl of Sandwich's" to make the sentence not repeat itself when it doesn't have to, thus the sentence becomes "The Earl of Sandwich introduced his favorite food, the sandwich". New nouns, on the other hand, can easily be made, and are constantly being added into the English language.