He hardly works at all

Students often confuse between 'hard' and 'hardly' thinking that the former is an adjective and the latter an adverb. Well, it can be quite tricky because if you make the following sentence, assuming 'hardly' to be an adverb, you may not be conveying what you intend to:

He hardly hit the dog with all his might.

You may think that he had used all his might and hit the dog forcefully. You are wrong as it is the reverse. This sentence simply mean 'He did not hit the dog with all his might at all!

The correct sentence should be 'He hit the dog hard and with all his might.

Now look at the next two sentences.

He works hard as he wants to earn more money.

He hardly works all day.

The second sentence is the equivalent of 'He does not work at all the whole day.