A pen is a writing tool originally made from a feather but presently a small tubular equipment containing ink used to make marks or write.
Kylie can't seem to find her pen. She said she dropped it on her desk before she went to the restroom.
A pin is a tiny device made of drawn out steel wire with one sharpened end and the other rounded into a head, used for fastening.
I still need a pin to fix my dress' collar.