Hard means with effort or diligently. It’s used when you want to express that someone is putting in a lot of work or effort to do something.
Example: „He worked hard to meet the deadline.„
(This means he put in a lot of effort to meet the deadline.)

Hardly means almost not or barely. It is used to show that something is done to a very small degree or almost not at all.
Example: „She hardly slept before the big presentation.„
(This means she barely slept or she almost didn’t sleep at all before the presentation.)

hard = hard
hardly = kaum