Difficult is used to describe NATURE or BEHAVIOUR. For example : 1. She was being difficult and didn't let the boys flirt with her. 2. The problem turned into a difficult situation and things became worse for him. Heavy is used to describe WEIGHT or MASS. For example : 1. The heavy bag weighed down the shoulders of the old man. 2. He carefully drove the heavy truck up the hill. Hard is used in the case of INTENSITY or MATERIAL. For example : 1. The human skull is very hard since it is made of tough bones. 2. Samantha gave a hard slap to the stalker on finding him. Hope it helps! :)