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:
Curry leaves are perhaps one of the highly popular herbs used in India since times immemorial. As the name suggests curry leaves are herbs which are added to curries, stews, soups, legumes, and many such preparations. Curry leaves are an integral part of Indian cooking style where many of the traditional and contemporary recipes are incomplete without curry leaves. Curry leaves are added not just for the flavour that they add, but they also stand to provide many benefits from health point of view.