Breakdown of Мой офис на первом этаже.
мой
my / mine
офис
office
на
on, at / by (transport) / to (a place) / to (in answer to)
первый
first (1st)
Questions & Answers about Мой офис на первом этаже.
The lesson is about numbers like один (one), so why does this sentence use первом?
Первом comes from первый, which is an ordinal number meaning "first". Just like in English, we say "on the first floor" instead of "on floor one".
Why do we say первом этаже instead of первый этаж?
Because we are talking about a location ("on the first floor"), we need to use the Prepositional case. The preposition на (on) changes the dictionary forms: первый becomes первом, and the noun этаж (floor) gets an -е at the end to become этаже.
Does первый этаж mean the ground floor or the floor above it?
In Russia, первый этаж is the ground floor (the floor at street level). This is exactly like American English, but different from British English where the "first floor" is one level above the ground.
Where is the word for "is" in this sentence?
Russian simply skips the verb "to be" (am, is, are) in the present tense. Мой офис на... literally translates as "My office on...".