Idę do sklepu- what case is this?
I thought it was locative, but I've been told the locative for of sklep is sklepie, if it's not locative what is it? And why? I'm quite puzzled... Any help greatly appreciated!

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Answer

It's certainly genitive.
Some prepositions can be a little bit tricky. One of those prepositions is "do". After "do" we always use nouns in genitive.

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Jordan is right, it's genetivus.

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It's Genitive with preposition 'do':
- Dokąd idziesz?  
- Idę do sklepu, do domu, do biura, do pracy, do mamy, do ciebie, do was, do kogoś, etc.

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Pretty sure it's genative. i know that after 'do' is always -u ending with masculine inanimate nouns.

That's all the info i can give you, really.  I'm trying to figure it out as well.

http://www.polish-dictionary.com/polish-genitive-case