Common nouns are parts of a group. Proper nouns name a specific unit within that group and are capitalized. The new word may be written as one word, with a space between the words, or with a hyphen . The best way to check how the word is written is to look in the dictionary.
Collective nouns are plural if the writer considers the members of the group individually.
Some nouns are irregular. Playing baseball is fun. Playing is the subject of the sentence . Mary went to her car. The pronoun refers to the noun that comes before the pronoun.