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.
- why it appears in some words (e. g. neuters, masculines, strong nouns, etc.) and not others (feminines, weak nouns, etc.);
- how it came to mark the plural in general and not just specific forms of the plural, e. g. the nominative and accusative plural,
- why there's no umlaut in singular forms, so e. g. umlauting of singular datives or singular genitives vs. non-umlauted nominative singulars;
- 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.