Where do you learn strength in Pokemon platinum version?

You can learn Strength in Pokémon Platinum by getting it from Roark in Oreburgh Mine.

Here's how:

1. Beat Roark: Roark is the Gym Leader of Oreburgh, located in Oreburgh Mine.

2. Defeat his Rock-type Pokémon: Once you defeat Roark, he will reward you with the Strength HM.

3. Add it to your Pokédex: You can now use Strength to move large rocks in the game.

Note: You can only learn HMs once per game. You'll need to use the Strength HM on your Pokémon to use the move.