My online Russian course gives me two examples of using что-нибудь. Он хочет что-нибудь попить. and Можно мне чего-нибудь попить? Why is the genitive used in the latter? When I tried to parse the sentence I would have expected accusative, not genitive. I see discussions of other uses of чего, but couldn't see how to apply those to this example.
You can use both, the Accusative (written speech) and the Genitive.(colloquial ).
Можно мне что‐нибудь/чего-нибудь попить?
In this particular sentence we can call it the Partitive Genitive which stands for a noun that denotes divisible substances, for example, foodstuffs like молоко "milk," and вода "water." These substances can be measured (e.g., литр воды), quantified (e.g., немного молока) or otherwise portioned out (e.g., кусок хлеба) - hence, the name and the basic notion of the Partitive Genitive.
One expects to use the genitive case with quantifying adverbs like много, мало, немного and in noun phrases where the head noun denotes a measurement, quantity, weight or portion, for example, стакан воды, чашка чаю.
The accusative signifies the item itself or the whole item. The Partitive Genitive signifies "some," "a part," "a little" or "a few."
Thank you, V.V. Now I understand the grammatical basis for this. I had learned the use of the genitive with quantifying adverbs, but had not thought of it here. I can absolutely see how you would want to use the partitive genitive when asking for something to drink or eat. Jan 28, 2017 at 17:52
Можно мне что-нибудь попить?As for the question Why, I can't explain, but I think you just need to remember such things, which can be done with reading etc. over some period of time.