1
He hit the ball very hardly.
He hit the ball very hard.
She studies very hardly before exams.
She studies very hard before exams.
Hard can be an adjective or an adverb. As an adverb, it means ‘with a lot of effort or force’: ‘She works very hard.’
Hardly has a different meaning. It means ‘almost not’: ‘I could hardly see anything in the rain.’
2
It was so noisy that we couldn’t hardly hear our own voices.
It was so noisy that we could hardly hear our own voices.
There were hardly no people at the concert.
There were hardly any people at the concert.
Hardly already has a negative meaning, so do not use it with another negative word. Use could hardly, not couldn’t hardly, and hardly any, not hardly no.
See NEVER (2)
3
I only have one question left, so I have hardly finished the test.
I only have one question left, so I have almost finished the test.
Hardly means ‘almost not’: ‘I have hardly started.’ Almost means ‘nearly’: ‘I have almost finished.’