Welcome to german grammar (GG)! This is not simple. Langenscheidt publishes a small grammar called "GG in a nutshell".
Briefly, adjectives that follow the noun are short and plain. Those before the noun without an inflected article must get an ending to match the noun in number, gender, and case. An inflected article relieves you of that requirement, since the article already matches the number, gender, and case.
In conversation, I just give up on this and count on my listener to pardon my ignorance.
This interested me so I looked it up. According to Wikipedia:
In the singular, viel is often left unchanged when it is not preceded by an article or determiner (e.g. Er hat viel Geld verloren). With articles/determiners it is declined like a normal adjective: das viele Geld.
In the plural, the adjective is usually declined either way (Ich habe viele Kinder, ich helfe vielen Menschen) However, it is sometimes left unchanged in the combinations wie viel (“how many”) and so viel (“so many”): wie viel Kinder or wie viele Kinder.
So in summary, "viel" if it's singular and without an article. Normal adjective rules with article or if it's plural (Nom Acc: viele, Dat: vielen, Gen: vieler).
O-umlaut is not ''oo'' but ''err'' as in ''To err is human.''
Mo''gen.....For english speakers, the spelling ''mergen" gives a surprisingly accurate german pronunciation. (That ''g' is hard, like ''get.' . German has no soft g.)
An example from german literature:: Wolfgang Gerta, (Go''the).
A southern accent makes it even better, killing the R but saving the vowel.
Actually, not at all. The "g" pronounced in "zwanzig" is still a hard "g" it's just that some native speakers will essentially have an "accent" on how they pronounce the "g" almost exactly like the "ch" is pronounced. For example, these two words: Mag - "like" Mach - "make" When pronounced, they sound exactly the same.
If you pronounce zwanzig as zwanzik, it is you who has an accent :)
Standard German pronounces the ending -ig as -ich. Listen to the model pronunciation at duden.de, for example: https://www.duden.de/rechtschreibung/zwanzig (click on the speaker icon under "Aussprache).
Your example of mag = mach is not parallel because it doesn't involve the ending -ig. (And for northern Germans who pronounce those two words the same, another aspect of that accent is that long vowels are sometimes shortened. In standard German, mag has a long vowel, mach a short one.)