1. Some might learn it faster and some slower. No - This should be Some might learn (it) quickly and some more slowly. (Adverbs are needed here)
2. Some might learn it faster(adverb needed or a comparison) and some slowlier.(slowlier is NOT a word)
Some might learn it faster than others.
3. Some might learn it easily and some with difficulty. (This is OK - but I would write
Some might learn it easily while others will have more difficulty.
4. Some might learn it easily and some difficultly. (difficultly is NOT a word)
I would write
Some might learn more quickly than others.
Use of hardly.
Hardly can �t be used as suggested here.
2. Some might learn it easily and some hardly.
(hardly isnot used this way. it is not the opposite of easily . It is not the adverb from hard/difficult. )
Usage:
Some learn a lot while some learn hardly anything.
We need more milk. There�s hardly any left.
She �s so short she can hardly reach the shelf.