Theory:

A common noun is a generic name for a person, place or thing in a class or group. A common noun is not capitalized unless it either begins a sentence or appears in a title. Common nouns can be concrete, abstract or collective.
A common noun is commonly used in speech and writing to perform many functions. The common noun serves to introduce or identify some general person, thing, idea or place. It names things according to common qualities or features. As a proper noun, it can also act as an object, a direct object, an indirect object, an object of a preposition or a predicate nominative.
Example:
People: mother, father, baby, child, toddler, teenager, grandmother, student, teacher, minister, businessperson, salesclerk, woman, man
Animals: lion, tiger, bear, dog, cat, alligator, cricket, bird, wolf
Things: table, truck, book, pencil, iPad, computer, coat, boots
Places: city, state, country, continent, coffee shop, restaurant, park, zoo
Ideas: envy, love, hate, respect, patriotism, pride
Job titles and general titles fall under the category of common nouns – attorney, actor, comedian, truck driver, sergeant, officer, secretary. However, if these become specific titles referring to a specific person, they sometimes become proper nouns.
  • Example:
    Attorney General William was appointed by President.
Here "Attorney" is the common noun, whereas "President" will be a proper noun.