Your Pokémon have the ability to learn different moves during your Pokémon journey. In Pokémon Brilliant Diamond and Brilliant Pearl, your Pokémon can only learn four moves at a time, forcing you to make them forget certain moves or not learn them as they level up. However, you may want to relearn these attacks later. You can do this with the Move Tutor. Here's where you need to go to find the Move Tutor in Pokémon Brilliant Diamond and Brilliant Pearl.
You will be able to locate the Move Tutor about halfway through the game before fighting the fourth Gym Leader, Wake. The Move Tutor's home is in Pastoria City. You will need to progress through the game to reach this point and defeat the third Gym Leader, Maylene.
When you reach Pastoria City, head to the south of the city and look for the house near the city water. It will be the closest house. Come in and talk to him. She will explain that he can teach Pokémon the moves they have forgotten. However, it does require Heart Scales as a payment. You'll need it if you want to use his services in Pokémon Brilliant Diamond and Brilliant Pearl.