declension of manch, solch, viel and others

Hey, dude, how are you? I’m studying the declinations of some words like viel, manch, solch, all… and the following adjectives. As a matter of fact, I understood quite easily some of the general rules (most of them I saw at DUDEN-Richtiges und Gutes Deutsch) but a doubt appeared through my studies: I can’t quite understand when to decline or not the words I mentioned before. I mean, I know how these words should be declined, but I don’t know WHEN they should be declined. I don’t really know if there are any rules or something, but if you could give me at least a hint, I would really appreciate :)

Auf Wiedersehen!

 

(asked by Filipe)