czegoś - An item or /something belonging to someone/somebody coś - something " Depends if your vague about it or not, and if you want to underline it belonging to someone or dismiss it. "Coś" is far more applicable and general while "czegoś" is used in a more specific situation or situations regarding the relationship somebody has with said item and who or how many possess it
With exceptions, such as a current student of mine, Ironside is correct. Then again, names, as with fashions, do go through phases:-) @Maf, even more diminuated "Tadziu" LOL
"I want something" = "(Ja) chcę coś" "I don't want something" = "(Ja) nie chcę czegoś"
"I (don't) need/use/listen to something" = "(Ja) (nie) potrzebuję/używam/słucham czegoś"
"I (don't) ask/fight for something" = "(Ja) (nie) proszę/walczę o coś"
"I (don't) pay for something" = "(Ja) (nie) płacę za coś"
"This is (not) good for something" = "To (nie) jest dobre na coś"
"I am (not) (some verbs of movement/motion) ahead of/after/behind/above/under something" = "(Ja) (nie) (some verbs of movement/motion) za/po/przed/nad/pod coś"
How then would you/one distinguish both the meaning and the usage of "czego" vs. "czegoS"? The compensatory "s with accent mark" is required here as elsewhere,
but I still don't understand why or how it is used. Please clarify:-)
Let's make it even more complicated. How about "cosik" - is it actually a recognized Polish word? It also means "coś" (or "a little something"), but may have derived from the Czech or Slovak language.