How do Pokemon forget hm moves on black and white?

You can't make Pokémon forget HMs in Pokémon Black and White. The HMs (Hidden Machines) are permanently learned by your Pokémon once you teach them, and there's no way to unlearn them in these games.

This is different from other generations of Pokémon where you can use the move "Forget" at a Move Deleter to make a Pokémon forget any move, including HMs.

So, if you want to teach your Pokémon a new move in Black and White, you'll have to decide if you want to keep the HM or forget the new move later.