Hard is both an adjective and an adverb. When it is an adverb, it means ‘needing or using a lot of physical or mental effort’. It goes after the main verb:
I studied hard for my exams but didn’t do very well.
We have worked hard all day.
The comparative and superlative forms are harder and hardest:
He didn’t get into the team this year. He’ll just have to try harder next time.
He’ll just have to try more hard next time.
Right, children, who has worked hardest today?
Don’t confuse hard with hardly. Hardly usually means ‘only just’ or ‘almost not’:
Every day I ran a lot and worked hard so that I would be prepared for the game.
Every day I ran a lot and worked hardly.