How to decide whether to use indefinite or definite articles?

Question

Very elementary question, and one would expect that I would be able to answer it by now since I've been learnign the language for over a year and a half. A begineer asked me a question which stumped me,

Meine Mutter faehrt naechste Woche in __ Urlaub

I argued that the answer should be "einem" as it would most probably be that the mother has gone on many vacations, and this is just one of many, hence an indefinite article. However, the right answer was actually definite article.

Is there a deeper logic which helps in deciding to use definite or indefinite articles, or is it case by case basis?

Answer 1

Well, you could say in einen Urlaub if you want to emphasize the fact that she goes on one of those many vacations.

But that's likely not what you want to convey. More likely you want to express that she goes on that particular vacation that is due. That's a definite vacation, thus in den Urlaub.

Finally, you could also say in Urlaub as this is a fixed phrase.

---

It's one of those questions even native speakers would get "wrong". But there is no wrong. It's completely dependent on context and your level of German. If your materials have a lot of such dimwitted trick questions, it's a good idea to dump them into Ablage, rund.