Breakdown of Мой брат работает в нашем новом офисе.
брат
brother
новый
new
мой
my (мой, моя, моё) / mine
наш
our (наш, наша, наше)
Questions & Answers about Мой брат работает в нашем новом офисе.
Why do we use нашем and новом instead of their dictionary forms, наш and новый?
Because they are describing a location. They are attached to the noun офисе (office), which is in the prepositional case to show where the action happens. Adjectives and possessive pronouns must always match the case of the noun they describe.
What are the exact ending rules for нашем and новом here?
Since офис is a masculine noun, its matching adjectives and pronouns must take masculine prepositional endings. Adjectives usually take -ом (like нов-ый changing to нов-ом). Possessive pronouns often take -ем (like наш changing to наш-ем).
Why does офис change to офисе?
The word в (in) followed by a location triggers the prepositional case. For most regular masculine nouns that end in a consonant, like офис, you simply add an -е to the end to make it офисе.
Why does "our" change to нашем, but "my" stays in its basic form Мой?