When can you use the hm strength in Pokemon diamond?

You can learn HM Strength in Pokémon Diamond after you defeat the fourth gym leader, Gardenia, in Eterna City.

Here's how to get it:

1. Talk to the old man standing near the entrance to the Eterna Forest. He'll give you HM04 (Strength) and tell you about a nearby Pokémon Center.

2. Head to the Pokémon Center and teach the HM to one of your Pokémon.

Important Note: You can only use HM Strength on certain boulders in the game.

Hope this helps you get your strength on!