If this is your first time playing, I strongly encourage you to play
normal at first. The game was built around normal rather than hard,
so normal mode is the proper experience.

The hard mode patch does not contain any main game data and is not
mandatory. You do not need to apply the hard mode patch in order to
play the game. Patch it onto the ROM after you apply the main Emerald
patch. I cannot guarantee game stability if you patch hard first.

Hard mode does not alter the level curve at all Instead, it

- Removes the ability to gain EVs from any source
- Hides natures (to encourage you to not worry about them)
- Disables using items such as Potions and X Attack in battle
-- Hold items are unaffected, and AI trainers do not use any
   items that you can't use.
- Disables Switch mode, so you do not get free switches for
  defeating an enemy puppet

The changes are intended to make you want to think more in battles
and accept things can faint instead of just using one or two puppets
through the entire game and healing them in battle when they faint.

It can't prevent you from grinding far above the levels of the opposing
trainers, so it's somewhat of an honor system. If you don't use items
in battle or you use Set mode normally, then hard mode probably won't
change much for you. You probably won't even notice its effect until
somewhere around the Fortree segment.

If these changes don't appeal to you, then don't apply hard mode.

Hard mode saves are completely compatible with normal mode, so if you
start in hard and don't like it then just resume playing in normal.