Learn Dutch: Waarom lopen jullie te klagen? - Why are you walking around complaining?

It can, but not necessarily! The structure lopen te + infinitive is used to show an ongoing, continuous action. While it often implies physical movement, Dutch speakers frequently use lopen figuratively to express irritation about a drawn-out action, much like saying "Why are you going around complaining?" in English.

Could I use zitten instead of lopen in this sentence?

Yes, absolutely! You could say Waarom zitten jullie te klagen? Dutch uses posture verbs (zitten, staan, liggen, lopen) to show that an action is currently happening. You generally pick the one that matches the person's physical position. If they are seated, use zitten. Both lopen and zitten are also used figuratively just to mean someone is busy doing something.

Why is the verb split up with jullie in the middle?

In Dutch questions that start with a question word like Waarom (why), the conjugated verb (lopen) must take the second spot. The subject (jullie) immediately follows it. The rest of the verb phrase, which is the te + infinitive (te klagen), gets pushed to the very end of the sentence.

How would this sentence change if I were only talking to one person?