Why doesn't "ninth" have an "e", like "ninety"?
Is it just because "ninth" has only one syllable? That wouldn't make sense, though, because saying "NINE-ith" wouldn't be worse than saying "NINE-e-tee". If we were used to "nineth", we would have no more trouble pronouncing it than "ninety" or "nineteen". Where did the "e" go, and why didn't it disappear from the other words with "nine" in them?
Old English
The Old English spellings for the number nine were nigon or nigen or nigan (see Dutch negen) - actually written "niᵹ[oea]n" with the old Irish "g".
For instance in "The coronation of Edgar [the peaceful]" (a poem from the Anglo Saxon chronicles, composed at the end of the... 9th century) one can read:
OE: Ond him Eadmundes eafora hæfde nigon ond XX
PDE: And Edmund's offspring had 9 and 20 [years]
Derived from nigon, you would find typically
- nigonhund ==> nine hundred :
- nigontig ==> ninety
and...
- nigonðe ==> ninth
It is probably fair to assume that the "-gon" part was unstressed and this explains why there are at least three variants nigon, nigen and nigan. The vowel of the second syllable was not pronounced distinctly and gradually faded away.
Middle English
Actually the Middle English spelling shows that it disappeared pretty quickly. Here are a few spellings from Middle English.
1225 Ancr. R. 328
Þe nieðe reisun is þis.
The ninth reason is this.
c 1357 Lay Folks Catech. 232
The neynd is, that we noght yerne our neghtebur house.
The ninth [commandment] is that we not covet our neighbour's house
In nine you need the final "e" to suggest a pronunciation of /naɪn/ as opposed to /nin/ (nin).
Just as in wine /waɪn/ vs win /win/.
So that the final "e" could not disappear.
Modern English
As for ninth, however, it looks like the spellings nineth and ninth competed for a while and that ninth eventually prevailed. I don't think there's any rhyme in /-inθ/ in English that would justify the expense of an extra "e", the role of which would be to avoid the confusion between nineth /nainθ/ and ninth /ninθ/.
Here is an example of nineth in Modern English.
1688 R. Holme Armoury iii. 190/1
The Knights of St. Stephen instituted in honor of Pope Stephen the nineth.
Also note: The same thing happened to transform eahtoþa into eighth: the unstressed "o" gradually became less heard and the spelling adapted.
Sources: OED and Oxford Dictionary of English Etymology 1992.
The only thing I can find that sort of covers this is in the wiki for ordinal indicator, it explains how to write them, but not why they are pronounced that way.
I think this will solve your question. I found it in the book "Vocabulary for the High School Student" by Harold Levine:
Attaching Suffixes to Words Ending in Silent E
When you add a suffix to a word ending in silent e, what happens to the e? Is it kept or dropped? Here are the rules:
Rule 1
Drop silent e if the suffix begins with a vowel,
EXAMPLES: blame + able= blamable; secure + ity= security; innovate + or= innovator
Exception A> If the word ends in ce or ge, and the suffix begins with a or o, keep the e.
EXAMPLES: service + able= serviceable; courage + ous= courageous
Exception B> acreage, mileage, singeing, canoeing, hoeing, shoeing
Rule 2
Keep silent e if the suffix begins with a consonant.
EXAMPLES: hope + ful= hopeful; profuse + ly= profusely; postpone + ment=postponement
Exceptions argument, awful, duly, truly, wholly, ninth