Why is vier in the beginning of vierzehn [ˈfɪrʦeːn] not pronounced the same as the standalone vier [fiːɐ]?
- "Rules" for pronunciations are merely descriptive not prescriptive.
- The pronunciation depends on the whole word.
- The numbers up until 20 were more often used than numbers greater than 20 when the German language developed. That's why the pronunciation of "vierzehn" could develop more independently from "vier" than for example "vierhundert". So, the word "vierzehn" got a pronunciation different from the pronunciation constructed out of the parts "vier" and "zehn".
- There are more examples, where the word differs from the construction of its parts:
- 11, 12: completely different
- 16: second "s" is dropped
- 17: second syllable is dropped
- One model could be the shortening of the first part of the numbers from 13 to 19, but there are only two numbers, where this is possible:
- vier → vierzehn: long "i" becomes short "i"
- sieben → siebzehn: two syllables become one syllable
- in all the other numbers there is only one syllable and the vowels are already short or diphthongs, which can't be shortened.
This is a very good question. I can only state assumptions. I think it evolved as a quirk because it's easier to pronounce. For example, the word 'fünfzehn' (15) is sometimes pronounced 'fuffzehn', and the word 'zwanzig' (20) is sometimes pronounced 'zwanzich' (very common in northern Germany).
Try to slowly pronounce the word 'vier' with a long 'i' and after that, pronounce it with a short 'i'. You will notice that the long 'i' sound takes considerably more effort to produce. For a clear, long 'i' sound, you have to open your mouth quite a bit, while for the short 'i' sound, it can stay shut. As Hackworth stated, it rolls easier off the tongue.
The interesting thing with 'vierzehn' is that its pronunciation quirk (the short 'i') is actually made official - dictionaries list it as the only correct pronunciation.
The word 'siebzehn' (17) is another interesting example. Myself and everyone I know pronounce it with a short 'i' sound as well, just like 'vierzehn'. But in contrast to 'vierzehn', dictionaries list 'siebzehn' as pronounced with a long 'i'. You see, there are quite some inconsistencies.
Vowel shortenings in closed syllables are a feature of some Middle German dialects that has found its way into the Northern German standard pronunciation. Other examples of such shortenings are the pronunciations of «gibst», «gedacht», or «gebracht» with short vowels.
The pattern is the same. Open vowels syllables have long vowels:
- sie-ben /ziː.bən/
- ge-ben /ɡeː.bən/
In closed syllables, these long vowels are shortened:
- sieb-zehn /zɪb.tseːn/
- gibst /ɡɪbst/
Due to the wide adoption of the Northern standard pronunciation in Germany, those pronunciations are finding their way into Southern Germany as well, where the long vowels are being used traditionally. The original pronunciation with long vowels remains common in Switzerland and, if I am not mistaken, in Austria:
- sieb-zehn /ziːb.tseːn/
- gibst /ɡiːbst/
Of course, there are other words where the Middle German shortening has not entered the Northern German standard pronunciation, that is, where the vowel remains long even though the syllable is closed:
- liebst /liːbst/
- schiebst /ʃiːbst/
I do not think there can be an answer to the question why some Middle German dialectal pronunciations have become standard and others have not.
I want to add, as a complement to Martin H.'s answer, where the pronunciation of 15 is evoked, the following map from the Atlas zur deutschen Alltagssprache
It actually depends on where you are and who you are speaking to. I pronounce vierzehn with a long i and vier with an audible r sound (not the vocalic r which sounds somewhat like a or the a-shwa). Therefore, my pronunciations of vier and the vier in vierzehn are pretty much identical.*
In other areas of Germany I have also heard the two occurances of vier being pronounced identically but differently from how I do it (using the vocalic r). I’ll listen up and ask around among my colleagues but I can’t really recall someone having different pronunciations of the two …
* Caveat: This is true if I don’t say viere instead of vier.
You can pronounce the beginning the same like four if you have not been understood the first time. Then you'd just say vier - zehn. But if you pronounce it in one word firzehn. The whole word sounds more balanced to me. Also I have a hard time pronouncing zehn after saying vier like if vier was standalone. There is just not enough air left. So I have to shorten the vier to get the ten out with enough pressure. Which alters the pronunciation of the syllable vier.
This would be a case of time-travel. The future is influencing the present in a way. ;)