Finite is pronounced ['faɪnaɪt] while infinite is pronounced [ˈɪnfɪnɪt]. So why is the vowel in the first syllable of 'finite' different from the vowel in the second syllable of 'infinite'?
OK, it's not because of Trisyllabic Laxing, I made an erroneous assumption which I deeply regret.
The answer is actually simple. As Luigi Burzio explains in Principles of English stress, the reason boils down to English stress patterns.
The diphthong [aɪ] (as in bite) almost never occurs in unstressed syllable. The diphthong [aɪ] has a systematic relationship with the short vowel [ɪ]. This relationship is also reflected in Trisyllabic Laxing; divine-divinity, derive-derivative and in drive-driven etc.
From this relationship, we can infer a general rule of thumb that [aɪ] will only occur in stressed syllables and when that syllable gets unstressed, [aɪ] will shorten to [ɪ].
Now when you prepend the prefix in- to finite, the primary stress moves to the prefix in- because it's a stress-bearing affix.
- In + f[aɪ]nite → inf[ɪ]nite
- In + p[əʊ]tent → imp[ə]tent
- In + m[aɪ]grant → imm[ɪ]grant
- In + f[eɪ]mous → inf[ə]mous
Another example would be cycle-bicycle:
- Bi + c[aɪ]cle → bic[ɪ]cle
There are exceptions, however. Luigi Burzio has explained all the rules and exceptions thoroughly in his book. One of the many exceptions is the prefix un- which doesn't take primary stress, for instance, unab[eɪ]ted.
Latinate words in English tend to be stressed on the penultimate (second last) syllable unless that syllable is short, in which case the primary stress falls on the antepenult (third last syllable) but not on preantepenult (fourth last) as far as I know. That's why impossible, implausible, incredible etc., are stressed on the antepenult.
Phonemically1 -ing is always /ɪŋ/. The vowel phoneme2 is decided by linguists to be /ɪ/, though it can be realised in many different ways.
Phonetically3, however, it's realised as [iŋ] in some dialects of English (particularly American); that is to say, the vowel [ɪ] raises to [i] due to the effect of the following velar nasal (nasalisation).
According to The Origins and Development of the English Language by John Algeo (p26):
[I]n the South [USA], the vowels [ɪ] and [ɛ], although distinguished in most environments (such as pit and pet), have merged before nasals. Thus pin and pen are homophones for many Southerners, as are tin and ten, Jim and gem, and ping and the first syllable of penguin. The sound used in the nasal environment is usually [ɪ], though before [ŋ] it may approach [i]. [Emphasis mine]
1. /phonemic transcriptions/ are language specific transcriptions i.e. the way dictionaries transcribe words. /They/ can have [many different realisations, depending on the speaker and accent]
2. ‘A phoneme is a mental image of all the various realisations of
one and the same sound.’ (Donka Minkova) For example, the phoneme /t/ is a mental image of many realisations such as [t], [tʰ], [t̚], [ʔ] etc., in some dialects of English. By contrast, if you substituted say b for t it would change the meaning (cf. tall and ball) so we would say that /b/ and /t/ are two distinct ‘phonemes’ in English.
3. [phonetic transcriptions] transcribe actual speech sounds i.e how people speak
Best Answer
TLDR
'South' and 'southern' have different vowels because they were affected by a phonological rule in Middle English called Trisyllabic Laxing (hereafter TSL). It was a process whereby a long vowel in a stressed syllable was shortened when it was followed by two or more syllables.
Explanation
Trisyllabic Laxing was a phonological rule in Middle English (and perhaps Old English) that shortened (laxed) a long vowel1 in a stressed syllable when it was followed by two or more syllables. It only applied to derived words (like divinity from divine and -ity) rather than underived words (like nightingale). At one point, it affected all the relevant words. Later on, it ceased to be a part of English phonology, though its remnants are still highly visible in Modern English. Some examples are as follows:
As you can see, there are at least two syllables after the stressed long vowels, so they got shortened because of TSL.
There's a fairly regular pattern of the vowel change in the words I mentioned above. It can be summarised as:
In Modern English, there are many exceptions such as words ending in -ness (e.g. mindfulness, loneliness etc) or later borrowings such as obese/ obesity etc.
The above changes happen in trisyllabic (three-syllable) or polysyllabic words, but 'southern' is not even a three-syllable word, why then is it pronounced with a short vowel?
The short vowel in 'southern'
The history of southern is more interesting. Wikipedia also mentions disyllabic laxing (when one syllable follows the stressed long vowel as in 'southern'), though the vowel in 'southern' is not the because of that. It is because of TSL.
The anomaly here is caused by historical sound changes. 'Southern' used to be a three-syllable word when Trisyllabic Laxing applied.
'South' was sūþ (or sūð)2 in Old English and it was pronounced /suːθ/. While 'southern' was sūþerne3 (or sūðerne) pronounced /ˈsuː.θer.ne/ ([ˈsuː.ðer.ne])
In Middle English, 'southern' was pronounced /ˈsuː.ðər.nə/. Then Trisyllabic Laxing applied to it and reduced the long vowel /uː/ to /ʊ/ (the vowel in put).
Then in Early Modern English, there was another sound change that caused the unrounding of the round vowel /ʊ/ in most varieties of English (exceptions being most accents of Northern England4 and Midlands) and changed it to /ʌ/ (vowel in strut). So the round vowel of 'southern' changed to /ʌ/.
'South' was pronounced /suːθ/ until the Great Vowel Shift, which changed the /uː/ vowel to /aʊ/ (vowel in mouth).
Footnotes:
References: