Our lecturer split it into
Institution: bei der, beim
Haus/Gebäude: im, in dem
But I mix things up. How can "in Büro" be correct (is it a Stadt?)? Can anyone give a concise guide, reference, article about it, please?
P.S.: I do understand, that in this case we use Dative and there gender forms are dem/der/dem/den correspondingly.