Who can use Hm strength in diamond?
Go down from Solecon Town then right. Go into the tower and climb to the top floor. Two ladies will be there. One will give you a Cleanse Tag. The other will give you HM Strength.
First if you mean strength you must beat the gym leader in pastoria city and she will give the HM strength. Then you teach to a Pokemon and you can use it. If you mean stealth rock you can teach it to a Pokemon if you have the TM for it or sometimes your Pokemon will learn it. I hope this is helpful
One of the basics of POkemon is that even if you have a HM (HM strength for example) you cannot use it until you beaten a certain gym. So, inother words. you wont be able to use it until you beat a certain gym. Whatever gym is next (One you havent beaten yet) will probably let you use it once you have beaten them.