"Why is the number falling?"
Translation:Hvorfor er tallet faldende?
I thought you weren't supposed to use the gerund like this (I am singing, it is falling), and only as an adjective? Why is it used here like the English gerund?
Because here it is an adjective present participle, not a gerund. Replace it by another to get the sentence structure.
"tal" is a number, either a specific one, or just numbers. "antal" is a total of something. You can replace "antal" with "tal" when talking about a specific total of something, just like here, but not if you want to tell what you are counting; for example, "antallet af cykler" cannot be replaced by "tallet af cykler", while "antallet" can be replaced by "tallet". I hope this makes sense, but it is a little difficult to explain.