You should be aware that not all adverbs of manner end in -ly. For example, the word FAST can be an adjective or an adverb – the spelling doesn't change.
On the other hand, some words end in -ly and are not adverbs! Examples are friendly and monthly. (The word 'monthly' may be used as an adjective or adverb.)
The word HARD can be used as an adjective and an adverb. The word HARDLY has a very special meaning – 'not very much'