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Chcę and chce [21]
In polish language there is something like partial genitive (dopełniacz cząstkowy). You use it, when you talk about indefinite part of a whole.
You use "chcieć" with partial genitive, when you want to say about indefinite quantity of something.
So when you say "chcę zimnego drinka" you mean by that you want some/ much/ glass...
You also use "chcieć" with partial genitive, when you talk about something abstract, e.g. chcę wolności.
You use "chcieć" with accusative, when you talk about something definite and considered as a whole "Chcę czerwony samochód".
That doesn't mean, that partial genitive is only applied to uncountable nouns. It is true in most cases, but not all.
For example "naleśniki" may be considered as countable and uncountable.
In your example "Chcę słodkiego naleśnika" - although you used singular noun, if you apply partial genitive, it will mean that you want some naleśniki (indefinite amount). Naleśniki may be considered here as a dish and you want part of it.
If you would say "chcę slodki naleśnik", it will mean that you want 1 sweet pancake.
Also, if you say "chcę wody", it will mean you want some indefinite amount of water. But, for example you are in shop and you ask for a botlle of water, you may say "chcę wodę".
See, not complicated at all ;)
Hope that will help:)