The Balrogs
 

I was just wondering if somebody could tell me how the Balrogs underwent the transformation from Maia to Fire-demons. Also why didn't sauron become a Balrog, as he was a Maia that was corrupted by Melkor.

The Balrogs were originally Fire-Maiar or something like that. Just as Ossë and Uinen are Water-Maiar. Sauron was a servant of Aulë; he had no business with fire and therefore did not become a Balrog.
Exactly how the transition from Maia to Demon works I do not know.

Greetings, gildor

I seem to recall somewhere in the reading that the Valar originally didn't have shape like we do, but they took on shape in order to converse with the Children of Iluvatar more easily. Remember in the Lord of the Rings, how Sauron is said to have been able to seem wise and noble for a long time? An example of this is when he was helping the Elves craft the Rings of Power. We also see in the tale of Beren and Luthien that he turns into a great wolf, and then into a snake, and atlast into a vampire.
The Valar and Maiar could change their appearance like this unless they expended so much of their power that they got 'stuck' with a certain shape. This eventually happened to both Morgoth and Sauron.

I'd say the Balrogs just chose to appear like that, for the intimidation factor I guess. To back up this theory, remeber Gandalf's account of his fight/fall with the Balrog, how he turned into "a thing of slime, stronger than a strangling snake."

Thanks a lot guys.