Difficult is the opposite of easy. It means that something requires lots of effort to do it. Hard can be the opposite of "soft" (such as a hard pillow and a soft pillow) but it can also mean "difficult."
For example, you can say:
The test was difficult. = The test was hard.
It's difficult to understand the teacher. = It's hard to understand the teacher.
The word hard is a little more informal, so it's better to use difficult in more formal writing.
The word hard can also be an adverb to describe doing an action with great effort or energy. We often use it with the verbs work and try:
Digestion Salt stimulates digestion by increasing digestive enzymes and so benefits low appetite and indigestion. It also works directly on the taste receptors and clarifies the sense of taste. Higher doses can work as a laxative or emetic. The yogic practice of sankhapraksalana and satkarma utilises the osmotic properties of salt to draw toxins out of the mucous membranes prior to purgation (Swami Satyananda). Lungs Salt has the ability to liquefy mucus and kapha in the lungs. It dislodges deep-seated phlegm and facilitates expectoration (Bhavaprakasa). Tumours Salt has the specific ability to dissolve masses, lumps and tumours in the body. As it clears the channels it removes obstructions. Care must be taken when treating these conditions with salt due to its tendency to cause water retention. Its iodine content (especially found in sea salt) plays a part in preventing goitre. Nerves Helps to settle the nerves by descending vata and calming the emotions.