The masculine form of "one" is "один," but it is "одна" and "одно" for the feminine and neuter, respectively. Why? In other words, why isn't it "одина" or "одино"?
My inquiry into this has been little more than a Google search, but after attempting various searches and taking a look at several pages to include those from textbooks, I have found nothing that addresses the fact that this word actually drops a letter to form the feminine and neuter forms. If I am not mistaken, this is actually called syncope in linguistics and refers to the loss of medial vowels in a word.