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:
Certain types of potatoes, particularly red and russet potatoes, contain high levels offlavonoid antioxidantsand vitamin A like zeaxanthin and carotenes, they can protect you against many types of cancer. Also, research at the Agricultural Research service has shown that potatoes contain a compound called quercetin, which has been proven to have anticancer and antitumor properties. Finally, the high levels of vitamin A and C both have antioxidant qualities that can protect your body from the devastating effects of cancer.