Do you use are or is when joining two singular subjects for example the book and pen in desk?

You would use are when joining two singular subjects with "and."

Here's why:

* The book and pen are in the desk.

The phrase "the book and pen" refers to two separate items, even though they are together. Since it's plural, we use the plural verb "are."

Remember: The verb agrees with the subject. Even though "book" and "pen" are singular, together they become a plural subject.