Hidden Machines, or HMs, are another kind of TM, but serve a new purpose. Unlike TMs, which simply teach your Pokemon a move, HMs are used in-game to help with your adventure. For example, the first HM that everyone knows is Cut. Although it is a very weak Normal-type move, it helps cut down trees in the game. This only happens when you have a Pokemon that knows Cut in your party. The same goes for all the other HMs, but instead of selecting them in the menu, simply press A when you are facing the obstacle. The game will ask you if you want to use that particular HM move.
Another HM we are all familiar with is Surf. Learnable by most Water-type Pokemon, Surf is a highly destructive move in battle, but is used to cross water in-game. A small note is that Surf now hits all surrounding Pokemon in battle, most likely to be a more fair attack.
In mid-game, the HM for Fly is received. Most players by now have a Flying-type Pokemon in their party, and Fly helps by letting them fly back to a town they have visited. It definitely helps with events such as finding outbreak Pokemon or getting your fossil of the day at Twist Mountain.
Strength is a good solid move that can be learned by a variety of Pokemon. Strength can be used when you are faced with an obstacle, such as a boulder. Your Pokemon can use Strength so your sprite can move the obstacles around. To save time, the game says that your Pokemon made it possible to move heavy objects around, so you don’t have to press A each time you want to move something.
Waterfall was added recently to scale waterfalls. To do this, you would also need a Pokemon with Surf so you can get into the water. However, you do not need Waterfall to go back down waterfalls. One important place you need Waterfall is to get to the Abundant Shrine, where you can find Landorus after you have captured Tornadus and Thundurus.
Dive allows you to find and explore new places by using it when you are surrounded by a dark spot in the water. In Undella Bay, you can use it to explore the ruins, which are filled with various plates that boost the power of certain type moves.
Some older HMs which were outruled in this game include Whirlpool, which allow you to cross whirlpools in the water, or Defog, to clear the fog in some Routes (Generation IV). Some TMs function as HMs, but are accessed at will using the menu.
Flash can make a cave brighter so you know where you’re walking, and Dig lets you escape a cave so you can end up where you entered. Of course, you don’t need to teach your Pokemon all these moves, but I would recommend Surf, Fly, and Strength, seeing as they are of some power and are definitely needed to save time in the game.