In platinum how do you learn back forgotten moves?

Theres a guy in pastoria city that will make ur Pokemon "relearn" its own moves.BUT you must pay him with a "heart scale". You can get em in solaceon in the house next to the poke center. Inside a guy will ask u to bring him a rare Pokemon and if u bring it to him he will pay u with a heart scale and some pokeballs.

hope this helped :)