Pokemon Brilliant Diamond and Shining Pearl, while underwhelming in some aspects addressed by players and critics already, were also pretty monumental in bringing necessary quality-of-life improvements to the Pokemon mainline games, which were long overdue.
One of these improvements were the changes made to the HM system. Usually, HMs were reusable items that could be used to teach your Pokemon a move that was necessary to clear some sort of obstacle in the overworld and progress through the game.
More often than not, these HMs would be pretty poor in utility and damage when used in battle, so teaching them to your Pokemon was something you had to scratch your head over. Apart from that, you had to go to a specific Move Deleter to forget said HM.
How do HMs Work in Pokemon Brilliant Diamond and Shining Pearl?
In Pokemon Brilliant Diamond and Shining Pearl, HMs are now categorized as TMs instead, which make them one time use. This means that you can teach them to your Pokemon and have them forget it whenever they want space for a new move.
Additionally, your Pokemon don’t have to learn the HM/TM for you to be able to overcome the obstacles in the overworld. All you need to do is get the TM/HM in your possession, and then beat the corresponding Gym Leader in order to unlock the ability to use the TM/HM in the overworld.
Just access your Poketch, open the Hidden Moves app, and select the HM you want to use. A random Pokemon from the wild will come to come to your aid and help you Fly, Surf, Cut down the tree in your path, and so on.