Final answer:
The correct rule for forming the plural of a noun in English is to add -s to the end of the noun, with some exceptions for plural nouns that already end in -s.
Step-by-step explanation:
In the study of grammar or the English language, the correct rule for forming the plural of a noun is to add -s to the end of the noun. For example, when you are talking about more than one book, you would say 'books.' Similarly, when you are referring to multiple cars, you would say 'cars.' It is important to note that there are some exceptions to this rule. For plural nouns that already end in -s, you would add an apostrophe (') after the -s to form the possessive plural.