How to say 'pen' in Spanish: Pluma. You try it. If you want to say something like 'can I borrow your pen?' you would say, "?Me prestos una pluma?" You try it.
There's also different ways to say 'pen' in Spanish, but I find that 'pluma' is the most commonly-used. There's also other ones, like 'esfero'. Go ahead, try that one; or 'boligrafo'. Try it.
Those are 3 different ways to say 'pen' in Spanish: Pluma, esfero, boligrafo.