Common nouns are words that name general items, people, or places rather than specific ones. For example, words like dog, city, and teacher are common nouns. They do not refer to a unique entity but rather to a class of objects or individuals that share similar characteristics. Common nouns can be singular or plural and are not capitalized unless they begin a sentence. They help us communicate about everyday things without needing to specify a particular instance. For instance, saying "I saw a bird" refers to any bird, not a specific one like a sparrow or eagle.