Salting bascially changes the original value and the comparison value with a known figure so the hashes can't be referenced to a lookup table, and so they can't be broken without knowing the salt value. Oh wait, we know the salt value now. Haha, that was easy.
Again, with the big exclamation of, "Everyone lock your doors, they might have gotten a copy of the KEY TO THE KINGDOM! *attachment: high-res picture of key to the kingdom.jpg*"
It doesn't really matter if you know the salt value. The salt doesn't have to be a secret; that's not the point of it. It's just so that a mapping of passwords to hashes can't be pre-computed ahead of time (which would then turn brute force attempts into a simple lookup). With a salt, you'd have to compute a table for each user separately, even if you know the salt for the each user, which is infeasible to do; and it's doubtful that any such tables already exist in the wild for any salted password on this forum, which, if you use a decently strong password, gives you ample time to go and change it wherever you used it before somebody cracks it.
That's because hash functions give very unpredictable result outputs by design. If you change even the slightest thing in the password it will hash to something completely different and unpredictable. And they are infeasible to reverse, so you can't just take the salt away from the hashed password after the fact. It's like trying to uncook a meal to get the raw ingredients back out again.