When to use bei, bei der, beim and in, im, in der?

Question

Our lecturer split it into

Institution: bei der, beim
Firma: bei
Stadt/Land: in
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.

Answer 1

Keep in mind that you often omit the article pronoun for a city's or a person's names (like Berlin, or Peter, etc.). You do use an article for a building's name, however.

Intitutions

• bei der Verwaltung fem.
• beim Amt obj.

Companies

• bei Google (Google is a name, therefore no article)

Cities

• in Berlin (a name again)

Houses/Buildings

• im Büro, in dem Büro obj.
• im Empire State Building (named building)
• auf dem Empire State Building (means physically on top!)

It is im Büro, which is short for in dem Büro to be explicit. Büro means office and is not a name. Reading the question I'm not sure whether this is clear, sorry if it is.