adverb
1
BAD : I tried hardly to remember where I had parked the car.
GOOD : I tried hard to remember where I had parked the car.
BAD : It was raining hardly and we all got wet.
GOOD : It was raining hard and we all got wet.
BAD : Society shouldn't punish these people too hardly.
GOOD : Society shouldn't punish these people too hard/severely.
◆◆◆
hardly = scarcely; almost not: 'It was hardly raining at all.' 'I could hardly believe my eyes.'
hard = (1) with a lot of effort: 'If you work hard, you're bound to pass.' (2) heavily or severely: 'The authorities are coming down hard on tax evasion.' Before a past participle, use severely for this meaning: 'If they are caught, they will be severely punished.'
2
BAD : It was so dark that we hardly could see.
GOOD : It was so dark that we could hardly see.
3
BAD : There were hardly no trees left, just bare rocky land.
GOOD : There were hardly any trees left, just bare rocky land.
BAD : When we arrived at the hotel, we couldn't hardly believe our eyes.
GOOD : When we arrived at the hotel, we could hardly believe our eyes.