Learn German: Das Büro ist im ersten Stock. - The office is on the first floor.

im is a combination of in and dem. Because the office is in a fixed location, we use the dative case. Stock is a masculine noun (der Stock), so its dative article is dem, which merges with in to become im.

Why is there an -en at the end of ersten?

In German, ordinal numbers like erste (first) act just like adjectives. Since the phrase is in the dative case (triggered by im), the adjective gets the standard dative ending -en.

Is im ersten Stock the floor you enter from the street?

Not in Germany! The ground level is called das Erdgeschoss. Der erste Stock is the floor above that. This matches UK English, but American English speakers need to remember that it translates to the American 'second floor'.

Could I use the word Etage instead of Stock?

Yes, die Etage also means 'floor' or 'story' and is very common. However, because it is a feminine noun, the grammar would change: the sentence would become Das Büro ist in der ersten Etage (since the feminine dative article is der).