WHEN TO Use Psalm or psalms?

Oh honey, let me break it down for you. When you're talking about a specific chapter from the Book of Psalms, you use "Psalm" followed by the chapter number, like "Psalm 23." But if you're referring to the entire book as a whole, you can just say "Psalms." So, remember, it's "Psalm" for one chapter and "Psalms" for the whole shebang.