what is the main function and sub-functions of a pen? Solution solution main function - main function of pen is to write anything. sub functions - to stab the eye of the school bully - pens are easily disassembled. Take the outer body and use it as a spitwad shooter - Stylus - Pointer - Trache-tube (artificial airway used in tracheotomies) - Emergency Axel for Bike - Snorkle - Straw - Candle-Holder.