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:
▶️ She works 16 hours a day. She works very hard.
▶️ He was trying hard not to cry.
In these cases, you cannot use “difficult.”