Replacing ない, the
auxiliary verb in standard Japanese that negates verbs, is
へん in 関西弁（かんさいべん）.
へん conjugates in the same way as ない, meaning that it connects to the
conjunctive form of verbs. Like ない,
へん may simply be translated as 'to not (A)'.
これ食（た）べへん。美味（おい）しくない。
I'm not going to eat this. It doesn't taste good.
せっかく海（うみ）に来（き）たのに泳（およ）がへんの？
Even though we came all the way to the beach, you aren't going to swim?
One of the primary differences to remember between ない and へん is that へん has an unchanging base, while the い in ない gets removed/changed depending on the conjugation type. This can be seen in the (casual) past negative form of verbs using へん.
欲（ほ）しかったパソコンが売（う）り切（き）れてたから、買（か）えなかった。
The computer that I wanted was out of stock, so I couldn't buy it.
欲（ほ）しかったパソコンが売（う）り切（き）れてたから、買（か）えへんかった。
The computer that I wanted was out of stock, so I couldn't buy it.
へん is a replacement for the
auxiliary verb ない, not the
い-Adjective ない. This means that
へん will not be used when negating
い-Adjectives. However,
へん may appear as a negative form of ある (written as あらへん). ある does not have a (casual) negative form in standard Japanese.
うちの犬（いぬ）は太（ふと）ってるけど小（ちい）さいから重（おも）くあらへんで。
Our dog is fat, but small so he's not heavy.
このテスト全然（ぜんぜん）難（むずか）しくあらへんやんか。
This test was not hard at all.