Learn Russian: Мой компьютер в новом кабинете.

In Russian, adjectives must match the case of the noun they describe. Because the computer is in the office, we use the Prepositional case to show location (after the preposition в). For masculine adjectives like новый, the Prepositional ending is -ом, making it новом.

If the sentence uses the Prepositional case, why doesn't Мой change to Моём?

Case endings only apply to the words playing that specific role in the sentence. Мой describes the компьютер, which is the subject of the sentence, so it stays in the basic Nominative case. Only the location phrase (в новом кабинете) changes to the Prepositional case.

Why is there an -е at the end of кабинете?

This is also because of the Prepositional case. For most masculine nouns that end in a consonant, like кабинет, you add an -е to the end when they are used to show location after the preposition в.

Where is the word for "is" in this sentence?

Russian usually skips the verb "to be" (am, is, are) in the present tense. So instead of saying "My computer is in the new office," you literally say "My computer in the new office."

Does кабинет ever mean a cabinet or piece of furniture?