Dictionary definition "Die Oper" using the dative case possibly

Question

Consider the following sentence:

Die Oper - Die Oper ist eine alte Kunstform, bei der die Schauspieler eine Geschichte mit Gesang erzählen. Begleitet werden sie von einem Orchester.

I managed to translate this as -

The opera is an old type of art, where the actors tell a story with singing. They are accompanied with an orchestra.

My questions concerns the use of "bei der".

I know that relative pronouns must relate to the case and the gender of the previous noun, hence the noun Oper has the gender "die."

I'm just confused about the case of the noun "Die Oper" when using "bei der".

I found that in the dative case, die turns into der, but I can't seem to work out why.

Unless I am incorrect,

Why is this case the dative case?

Answer 1

I would change your translation slightly to the following:

The opera is an old type of art, in which the actors tell a story (by) singing. They are accompanied by an orchestra.

So "bei der" means "in which."

"Der" has many uses, but in the feminine dative, it means "which."

"Bei" is a bit trickier. In English, we say "in which," but German uses "in" for things that are physically inside something else. In this case, the "in" is "metaphorical." Given this context, the correct German preposition is "bei," (English "by") rather than "in." The noun or pronoun accompanied by the preposition "bei" takes the dative case, which is why it is "der."

Answer 2

I don't know how good you are at German reading comprehension, but if you are decent, this is a good summary: https://mein-deutschbuch.de/praepositionen-mit-dativ.html

While this may not be a fully satisfying answer to "why" in the deeper sense, there are essentially a number of prepositions always requiring the Dativ case. Those are: ab, aus, bei, mit, nach, seit, von, zu

Answer 3

German articles can also serve as relative and demonstrative pronouns.

The dative feminine singular article is der:

Bei der Oper ...

The same goes if der is used as a relative pronoun:

In der Oper, bei der ..

So der can be an article, a demonstrative pronoun, or a relative pronoun and it can be feminine singular dative, feminine singular genitive or masculine singular nominative.

Answer 4

If you want to make it sound natural, and not "translated", then write:

Opera is an old artform, in which the actors tell a story by singing it. They are accompanied by an orchestra.

There is not always a one-to-one relation between English and German prepositions. In this case, in which is the correct form.

German bei usually means near, at, close to, and with, depending on the context. Personally, I would not use it here. I would use: "eine Kunstform, in der ...", but in such constructs, bei can be idiomatic.

Bei is used with the dative case.