What is the difference between the plural forms, e.g. pennor and pennorna or maskiner / maskinerna?

Question

Answer

en penna - a pen
pennor - pens
pennan - the pen
pennorna - the pens

en maskin - a machine
maskiner -  machines
maskinen - the machine
maskinerna - the machines

Answer

pennor is the plural, indefinite form of penna
and pennorna is the plural, definite form of penna

en penna: a pen
pennan: the pen
pennor: pens
pennorna: the pens

It is the same to the latter case.