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Adjective endings and masculine-personal in the Polish language [22]
I'd like to add to Koala's explanation my 3 grosze.
If you're saying: 'chcę coś' (biernik) you generally mean that you want it as a whole and you're not saying what is it needed you for.
On the other hand, saying 'chcę czegoś' (dopełniacz) usually indicates a part (or a portion) of it or suggests your intention of the usage (usually the most typical usage for given thing).
Some examples:
You're entering a store and saying:
Poproszę wodę. -> you mean that you want to buy a bottle of water.
Poproszę wody. -> you mean that you want to DRINK some water, e.g. because you're feeling bad
You're asking someone:
Potrzebuję młotek -> you mean you want someone to give you a hammer
Potrzebuję młotka -> you mean you're doing something that requires using a hammer to do it
a girl enters a room full of people and says:
potrzebuję mężczyznę -> she means a man (not a woman) is needed
potrzebuję mężczyzny -> she means she want's a man to... (so boys, get prepared if she's pretty, and beware otherwise ;-) )
Cinek