Go to the move rememberer, who will teach your Pokemon level up moves that it has forgotten in Gens 3, 4, and 5.
Gen III: Two Island, Fallarbor Town
Gen IV: Blackthorn City, Pastoria City
Gen V: Mistralton City, PWT
Yes. At the move rememberer. He'll teach a Pokemon a move it has forgotten from Level up in exchange for a Heart Scale.
Yes, in HG/SS, if you give the Move Rememberer a Heart Scale, you can have your Pokemon remember a move they have forgotten or can learn by level up. But, they cannot remember TMs or HMs they have forgotten. They can re-learn Egg moves