"Zijn zij in de hoofdstad aan het fietsen?"
Translation:Are they biking in the capital?
I put, "Are they in the capital biking?" I suppose this isn't the best way to say the sentence, but as a native English speaker I hear and say sentences like this all the time. Is this incorrect?
I will leave the answer to if your sentence is correct to a native speaker.
However, what I do know and have come across looking at it is that it that the location of the place in the sentence is unusual and does not follow the guidelines that are used to teach people English.