Here's why:
* Verb: A verb describes an action or a state of being. "Borrow" describes the action of taking something temporarily with the intention of returning it.
Let's look at some examples:
* Verb: "Can I borrow your pen?"
* Verb: "She borrowed a book from the library."
There is no adjective form of "borrow."
If you want to use a related adjective, you could use:
* Borrowed: "This is a borrowed book." (meaning it belongs to someone else)
* Loaned: "She gave me a loaned bike." (meaning it was temporarily given to her)