How do you push bolders in Pokemon emerald?

You can't actually push boulders in Pokémon Emerald. There are no boulders in the game that you can move.

It's possible you're thinking of a different Pokémon game, or perhaps you're mixing up the mechanics with another game altogether.

If you can tell me which part of the game you're referring to or what kind of boulder you're trying to move, I may be able to help further!