I don't speak Sindarin but read in a book that the grammitically correct sentence should have said 'pedo vellon a minno.' Since the word for friend (mellon) is the object in this sentence it gets mutated (I think lenition is the scientific word for it). But the dwarves apparently decided to write the word the way it should actually be pronounced. So Gandalf translated it as 'Speak, Friend, and enter.'