What did we gain in return for the loss of phonemic vowel length from Old English?

Yes, there is a connection between losing one phonemic property and gaining another. Most approaches to phonology conceptualize words as having double lives: on the one hand, they are made of a particular sound sequence which you have to pronounce correctly; on the other hand, the sounds in sequences are only recognized as discrete parts because they contrast with other sounds.

This property of phonology was termed double articulation by the French phonologist Martinet. One also speaks of form vs. substance: form being properties of speech sounds which they have by virtue of being in contrast with other sounds, or undergoing meaning-preserving alternations involving other sounds, and substance being the physical phonetic details of their pronunciation. (Quite confusingly, one also hears of function vs. form, where form coincides with substance from the other, roughly equivalent, dichotomy.)

These two factors frequently come into play in the evolution of vowel systems. To characterize it approximately: Consider a set system of 14 vowels, with 7 basic vowel qualities and two lengths. For such a system, the pertinent formal properties are that each vowel is contrastive with every other, and that the vowels may also be divided into pairs (e.g., e and e:) on the basis of meaning-preserving alternations. The fact that the pairs are differentiated by length and not by quality instead relates to the substance.

When a language undergoges sound change such that the phonetic substance is altered, but the formal relations between the sounds are preserved, it is usually referred to as transphonologization. Such a process is quite common historically, because languages do have a tendency of conservatism in form, if not in substance.

For further reading and numerous examples, consider a 2008 paper by the phonologist Larry Hyman and references included therein. For further reading treating English phonology more specifically, see various studies by Roger Lass, esp. English Phonology and Phonological Theory, and Old English Phonology.

(first draft, part 1, to be edited)

You have asked so many questions, and many of them don’t have simple, irrefutable answers.

So, I’ll try to address some of your questions below - immensely simplifying things and ignoring exceptions and minor cases.

At first, some caveats (the following is mostly based on Hogg 2008).

• Vowel length was rarely marked in the Anglo-Saxon manuscripts.

• Another problem is that there was considerable dialectal variation in OE. When we talk about the OE sound system, we actually mean one dialect, Late West Saxon.

• Another problem is that “the core antecedent” of Standard English wasn’t West Saxon at all – Hogg argues that West Saxon had a marginal influence only.

• Also, since there are no sound recordings of Old English speech, all we can do is to reconstruct the OE phonemic system. And, as usual, there is no consensus among linguists on many issues there.

Hogg 2008 reconstructs seven short vowels and seven long vowels for OE.

Q1. How do we know that there were short and long vowels in OE?

Q2. Were OE short and long vowels also quantitatively different, like in Latin?

Since there is no hard evidence – we have no recordings or native speakers of Old English – we don’t know for sure.

However, Hogg 2008 and Lass 2008 argue that short and long vowels began to diverge qualitatively before the end of OE.

Q3. Was vowel length sensitive to postvocalic environment in OE?

Lass 1994 argues that vowel length was free in OE. However, in late OE and early ME, things changed:

• homorganic lengthening: OE cild (hence, child);
• pre-cluster shortening: originally, before clusters of three consonants, e.g. OE cildru (hence, children); later before clusters of two consonants, e.g. OE cepte (hence, kept) vs. OE cepan (no shortening, hence keep), or OE mette (met) vs. OE metan (meet).

Q4. What about stress in OE?

It was always on the root. Noun and adjective prefixes were also stressed, unlike verb prefixes, which were usually unstressed (for more complex cases, see Lass 1994: 92). Hogg 2008 reminds us that what we usually know about OE stress is mostly based on poetry, and there is good reason to believe that in normal speech stress could have worked differently.

Also note that long vowels were always stressed in OE - in unstressed syllables, long vowels got reduced.