HMs are the centerpiece of any Pokémon game, and you'll collect them in Pokémon Brilliant Diamond and Brilliant Pearl. These are useful for removing obstacles blocking your path in the game. Previously, you had to teach a Pokémon on your team MN so that it could use it in the world, but it works a little differently for this game. Here's what you need to know about how HM works in Pokémon Brilliant Diamond and Brilliant Pearl.
You will use your Pokétch to use your HMs in Pokémon Brilliant Diamond and Brilliant Pearl. They will unlock as you progress through the game, so you'll need to unlock them as you continue your Pokémon journey. Fortunately, there is no need to teach him your Pokémon. Instead, when you receive the HM, you will also receive a MT of the attack that will allow you to use it in combat, but the HM will remove obstacles or allow you to travel on water.
When you're ready to use an HM, bring it to your Pokétch, while holding the R Button on your Nintendo Switch. You do need a specific Gym Badge to use it, though. For example, Smash Rock HM can only be used in the world after defeating the first Gym Leader in Oreburgh City. If you haven't, you won't be able to use the HM Rock Smash to smash boulders. The more MN you receive, the more Gym Badges you will need to collect to use them in the world.