The word hard works as an adjective when it modifies a noun.
That was a hard test.
You took a hard fall down the stairs.
The word hard acts as an adverb when it modifies a verb.
The team played hard but they still lost.
That professional baseball pitcher throws hard.
Adding –ly to an adjective will make the word an adverb.
slow becomes slowly
That is a slow train.
The train travels slowly.
In this case, the words slow and slowly have a similar meaning – the opposite of fast.
Things don’t work the same way when we use hard and hardly.
Hard means “not soft” as in hard concrete.
Hard means “difficult” as in a hard test.
Hard means “powerful and strong” as in he throws hard or he plays hard.
Yet when we put -ly onto hard, we get a completely different meaning.
The word hardly doesn’t mean “not soft,” “difficult,” or “powerful and strong.”
The word hardly means “scarcely” – and it is used to show that something is true in a very small way.
You have eaten hardly any of your dinner.
I’ve gotten hardly any sleep with all that noise.
Sam had hardly gotten home before the rain started.
Look at these sentences:
Bob tried hard.
Bob worked hard.
In each sentence, hard means that Bob gave good effort.
Now look what happens when we change the adverb:
Bob hardly tried.
Bob hardly worked.
In these sentences, hardly means almost not at all.
The adverb hardly has a completely different meaning from the adjective hard and the adverb hard.