Development of umlaut to mark plural nouns

Question

I'm looking for information on how umlauting became a plural marker - whether literature recommendations on it or just a short explanation, anything goes, note I am not in any university though.

In particular;

1. why it appears in some words (e. g. neuters, masculines, strong nouns, etc.) and not others (feminines, weak nouns, etc.);
2. how it came to mark the plural in general and not just specific forms of the plural, e. g. the nominative and accusative plural,
3. why there's no umlaut in singular forms, so e. g. umlauting of singular datives or singular genitives vs. non-umlauted nominative singulars;
4. how and why it spread to so many words that did not originally have any umlaut-triggering vowels whatsoever

I'm sadly not in any kind of university, so it is much more difficult to come by any information, however I'm willing to scour the internet for hours to find obscure papers if need be. Yes, I do speak German, so any information in German would be fantastic too.

Answer 1

Da Sie Deutsch sprechen, wähle ich den bequemen Weg.

Zwei Gegenbeispiele zu 1. (weibliche Substantive):

Kuh (sg) / Kühe (pl), Tochter (sg)/ Töchter (pl)

Ich denke, es ist schwer etwas weiblicheres zu finden als eine Tochter :-)

Sie suchen offenbar eine Regel für den Umlaut. Mein Eindruck ist, daß es keine gibt. Ich bin natürlich nicht der geeignetste, das zu beantworten, denn ich habe meine Mutterspache ja nicht nach Regeln gelernt, sondern nach dem Motto: "Es ist, wie es ist." Wenn es eine Regel gibt, kenne ich sie nicht, denn ich brauchte sie nie.

2. Pluralbildung und Kasusbildung scheinen mir unabhängig voneinander zu sein.

3. Es gibt auch Umlautbildungen beim Singular. Die Beispiele, die mir einfallen, betreffen nicht den Numerus, sondern machen aus einem Adjektiv ein Substantiv, für das die betreffende Eigenschaft charakteristisch ist:

hohl / Höhle, groß / Größe, gut /Güte, flach / Fläche

und viele andere mehr.

Answer 2

Summarizing Zwischen Konservierung, Eliminierung und Funktionalisierung: der Umlaut in den germanischen Sprachen, Chapter 2.3:

For masculine and neuter nouns, there were the i class (Umlaut and -e) and the iz/az (Umlaut and -er) that had umlaut in plural. However, the more common a class had no umlaut. In German, umlaut became the most prominent marker for plural. Especially in words like "Apfel-Äpfel", where final "-e" was removed quickly, it was the only marker. This is different from other Germanic languages, which either removed umlaut (English) or did not give it this function (Swedish).

Since umlaut was analysed as plural marker, it was subsequentially removed from singular forms and used analogously for plural forms that originally did not have umlaut in plural (the a class). The Umlaut + "-er" plural was also applied to many more nouns than originally, for example "Mann-Männer".

Feminine nouns, which usually have the "-en" plural, were usually not affected, except for e.g. "Mutter" and "Tochter".

Regarding your questions:

1. Masculines and neuters had declension classes with this pattern, female nouns not. The originally weak noun "Hahn" ceased to be weak and now has umlaut: "Hahn, Hähne". There just was no weak declension with umlaut.

2. 3. The declension of nouns was regularized and umlaut was coupled with number, so umlaut in singular vanished. I think all plural forms had umlaut if accusative / nominative had one.

3. by analogy and because the plural is then marked better. German prefers singular and plural to be different.

Regarding nouns with umlaut even in singular: There are 3 main sources:

1. The "-ia" declension class, for example "Käse"
2. The "-ī" declension class, containing many abstract female nouns such as "Höhe", "Höhle".
3. Suffixes that trigger umlaut, for example "Jäger", or nouns formed from other words, like "Ärger" from "ärgern".

Those examples are taken from Wikipedia on Old High German declension.