Learn German: Sehen Sie das Büro? - Do you see the office?

In the accusative case, only masculine nouns change their article (from der to den). Because Büro is a neuter noun, its article stays das. Feminine (die) and neuter (das) articles look exactly the same in the accusative case as they do in the nominative case.

Why does the sentence start with the verb Sehen?

This is how German forms "yes/no" questions. You simply swap the subject (Sie) and the verb (sehen). A regular statement would be Sie sehen das Büro (You see the office), so moving the verb to the front makes it a question: Sehen Sie das Büro? (Do you see the office?).

Why is Sie capitalized here even though it is not at the beginning of the sentence?

In German, the formal "you" (Sie) is always capitalized to show respect, no matter where it appears in a sentence. If it were written with a lowercase s (Sehen sie das Büro?), the sentence would mean "Do they see the office?" instead.