What is the difference between a singular noun and a plural noun when used in a comparative sentence?

Singular Noun

• The book is the one I'm looking for.
• The child is the one who broke the window.

Plural Noun

• The books are the ones I'm looking for.
• The children are the ones who broke the window.

As you can see, the plural noun takes the plural verb and the plural article, while the singular noun takes the singular verb and the singular article.