There's an English joke that goes something like this:
Boss: What are you doing?
Employee: I'm working on this project.
Boss: Really? Are you working hard or hardly working?
In order to understand the joke, we need to look at the difference between hard and hardly. In English, most adverbs are created by adding -ly to an adjective. For example:
◆ quick → quickly
◆ slow → slowly
◆ careful → carefully
However, there are a few special adjectives that don't add -ly when they become adverbs. For example:
◆ far → far
◆ fast → fast
◆ hard → hard
As an adverb, hard often means to do something with a lot of energy, effort or intensity. Some examples are:
◆ He is studying English very hard. (= studying a lot)
◆ She was laughing hard during the movie. (= laughing very energetically)
Hard can be used with both positive and negative verb forms.
◆ He is working hard to save money.
◆ He is not working hard to save money.
Hardly is also an adverb. It has a few different meanings, but often it is used to mean "almost" or "not much." For example: