"They are ill. That is why they are at home."
Translation:De er syke. Derfor er de hjemme.
This is called subject-verb inversion. The verb must always take the 2nd position of the clause. This is easily noted in sentences that begin with a location or time/ "I dag, regner det mye." (Today, it rained a lot.) Notice how the verb "regner", takes the spot before the subject "det".