What is the sentence on pen as an adjective?

You're asking for a sentence where "pen" is used as an adjective. It's a bit tricky because "pen" is usually a noun (a writing instrument).

Here's how we can make it work:

"She used a pen-shaped bottle opener."

In this sentence, "pen-shaped" is a compound adjective that describes the bottle opener. The word "pen" is part of this adjective.

Let me know if you'd like more examples!