To jest dom means both "this is a house" and "this is the house", right ?
Technically yes, but for 'the house' often it would make more sense to say 'ten dom' (this house, this house that we were talking about, etc.)
thanks for your answer. I was a bit confused about how to express those both things in polish, which have different meanings .But I gess I got it now, even though I still have to get a feeling for that, that´s the challenge :)
My friends sometime use "domu" "idzisz do domu?" Is this correct and if so is it in accusative case only?
It is genitive. Go to... is do + genitive (preposition do always requires the genitive).
Accusative for dom would be just dom (Widzę dom)