Why does the word "один" drop the "и" for its feminine and neuter forms?

Question

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.

Answer 1

This one is actually non-trivial. As @shabunc mentions, at a certain moment the root (which was originally один-) started to exhibit alternations between и and the ultrashort ь. However, if ь simply won over и, we would have something along the lines "одён - одна - одно" (which Zaliznyak's system in "Russkoe imennoe slovoizmenenie" actually predicts, he needs a special rule to change одён to один when going back from the abstract view ("условный вид")). So the root один was retained in nominative singular (and in some derivatives like одиночный, одинокий) but root одьн was chosen for declension (and for some other derivatives like однажды, однако - notably never in strong position though), leading to an almost unique situation of having a stressed fleeting и (only яиц has the same, but there one can at least point at the neighboring j - which is not quite enough either, compare заём - not *заим).

Also, I need to mention that historical deletion of ь/ъ is not conditioned by stress (see Zaliznyak 1985, ch. 3) but only by segmental context.

Answer 2

Well, in Old East Slavonic - which is in Russian called more often древнерусский - following forms are attested: одинъ / одьнъ, одина / одьна, одино / одьно. Note that here "ь" is not a soft sign but a ultrashort vowel of course, so called front yer). The 'ь' form is a later development, thus "и" was reduced to 'ь'.

So this forms were exactly like you'd rather expect them to be: logical and predictable ) And from historical standpoint the reduction of "ь"/"ъ" in unstressed positions a very common phonetic process in Russian evolution - there are hundreds, if not thousands examples of such reduction, like правьда -> правда, чьто -> что, кънига -> книга.

However, I honestly think that such questions are better to be asked on https://linguistics.stackexchange.com/ - since this process hadn't happened in modern Russian (which is our main focus) but actually preceded its formation.