Learn Dutch: Ruimen jullie het kantoor op? - Are you tidying up the office?

The full verb is opruimen (to tidy up). It is a separable verb. In a present tense main clause or a question, the verb splits: the conjugated part (ruimen) takes the normal verb position, and the prefix (op) moves to the very end of the sentence.

Why does the sentence start with Ruimen instead of a word like "Are" or "Do"?

Dutch does not use helping verbs like "are" or "do" to make yes/no questions. Instead, you just swap the subject and the verb. So, Jullie ruimen op (You tidy up) becomes Ruimen jullie op? (Are you tidying up?).

What exactly does jullie mean here?

Jullie is the plural form of "you" (like "you all" or "you guys"). If you were asking just one person, you would use jij or the formal u. That would change the sentence to Ruim jij het kantoor op? or Ruimt u het kantoor op?

How do I know to use het kantoor instead of de kantoor?

The word kantoor (office) is a neuter noun in Dutch, meaning it requires the definite article het. Because there are no easy rules for guessing this, it is always best to memorize a new noun together with its article, de or het.