In the Spanish grammar book I study there was a task to translate the following sentence to Spanish: "I prepare dinner".
I answered: "Preparo una cena".
Correct answer: "Preparo la cena".
Why do we need to use definite article here? In my understanding, if the dinner is not ready yet, you can't refer to it with the definite article, you're preparing some dinner.
I know that in English we don't use artilcles with the names of meals.
Is there some grammar rule I'm missing in Spanish?