I recently noticed that I am unable to recognize when a vowel shift should occur in going from the noun to the adjective. I wrote vielzählig when I believe it should have been vielzahlig. On the other hand, a random occurrence is a Zufall but if you do something by accident it is zufällig. So in one case you get the vowel shift and in the other you don’t.
These particular instances came up for me in the Yiddish context, but I think the same thing applies in German. I’m just wondering if there is some succinct rule that would guide me to know when to make the vowel shift and when not to?
EDIT: Hendrik points out that vielzahlig does indeed take the umlaut: my mistake, I somehow misread my German references. And although we almost always follow the German pattern in these things, here the Yiddish dictionary (Weinreich) definitely gives the unshifted form. In any case, there are other adjectives e.g. ehemalig which eschew the umlaut in both languages.