"Hard" we can use in lots of different ways, but it usually means "strong," "difficult," or "intense." - Ow! That ball's hard!

Calculus is hard!

That was really hard work.

"Hardly" is an adverb, and it can mean "almost not."

This is too small. I can hardly read it.

We can use it with "any" to talk about quantity.

For example: There's hardly any water in this cup.

We can use it with "ever" for frequency.

For example: I hardly ever play basketball.

To help you remember: It's hard for me to play tennis because I hardly ever train.

I can hardly walk afterwards because I've been running hard.

And I often get hurt because the ball's hard, and there's hardly any time to get out of the way!