The Move Relearner in Pokémon Brilliant Diamond and Shining Pearl is a great tool. You can use it to teach Pokémon old moves. As you level up a team member, they learn new moves. Sometimes you lose an old move to learn the new one. You can then use a Heart Scale to bring back that old move. The Relearner can also help with teaching new moves to a team if you need to adjust to a new meta-game in a hurry.
You can find the Move Relearner in Pastoria City. It will be the house just east of the Gym, look for the berry trees in the back of house. Go inside their house and select the Pokémon you wish to teach a move to. The move list will be different depending on the choice you make. Pick a move the synergizes well with the monster you’re training.
The most useful thing you can do besides the Move Relearner, a Heart Scale can help you a bit. These items can help monsters relearn old moves that they’ve forgotten.
And if you’re trying to finish the dex, you’re going to need to hunt evolutions as well as moves. Aside from evolutions and other stuff, there’s a lot of things you can do to complete the Dex. Changing a Pokémon’s Nature is one of many ways you can breed for success in this game. And if you’re hunting Shinies, that’s a lot of extra work too. If you’re a shiny hunter, there’s a good chance you are using something like the Masuda method to get it done. That means you probably have a lot of excess monsters in your boxes. If you want to make management of your teams easier, get that Box Link.
There’s a lot to learn about this game. You might want to learn a bit about Affection bonuses. There are also tons of evolution stones and other items you need to hunt for. Pokémon swarms are back as well, adding another layer to capturing rare monsters. When you want to explore the Grand Underground, you’re going to need a lot of Digger Drills.