Does Young Goodman Brown dream the Black Mass or actually see it? How do you know? Does it make a difference in your interpretation of the story?
We cannot know for sure whether Goodman Brown actually saw the Witches' Sabbath or if he dreamed it because there are no absolutely definitive clues that establish the truth one way or the other. However, it doesn't actually matter whether it was real or a dream. We do know that Brown left his home, abandoning his wife, Faith, symbolic of Christian faith in general. On his way into the forest, he thought to himself,
Methought [...] there was trouble in her face, as if a dream had warned her what work is to be done to-night. But, no, no! 't would kill her to think it. Well; she's a blessed angel on earth; and after this one night, I'll cling to her skirts and follow her to Heaven.
In other words, then, Brown makes the conscious decision to leave his faith behind, to go into the woods for some admittedly dark purpose that he doesn't even want his wife to know about, for one last night. He abandons God and all his Christian precepts with the intention of resuming them the next day, but this is not how religious faith works. One cannot simply pick and choose those moments when one will practice one's faith and then abandon it when it becomes inconvenient. By abandoning his faith, embodied by his wife, Faith, Brown turns his back on God, so whether he actually sees the Black Mass or merely dreams it, he has already condemned himself.
Whether Young Goodman Brown dreamed the night in the forest is a question the reader must decide for himself. To me, it doesn't matter whether it's a dream or not because Young Goodman Brown believes in his heart and soul that it did happen, and this changes the rest of his life. Brown makes the decision to give up on humankind and believe his wife and friends are evil, and he spends the remainder of his life alone, feeling that evil has won out over good. The Puritans believed that all people are evil because of the "original sin" committed by Adam and Eve in the Garden of Eden. It is only through belief in God and living a pure life that man can overcome evil. Brown's night in the forest, whether a dream or reality, takes away his belief in humanity. The Black Mass in the forest forces Brown to see himself as just one evil part of a corrupt race of sinners.