Question 1

Why is welche used here, and what does it refer to?

Accepted Answer

In German, welche acts as an indefinite pronoun meaning "some" or "any" when it replaces an unspecified plural noun or an uncountable mass noun. Here, it replaces the mass noun Pappe (cardboard) so you don't have to repeat the word.

Question 2

How do we know to use the form welche rather than welchen or welches?

Accepted Answer

The pronoun must match the gender and case of the noun it replaces. Pappe is a feminine noun (die Pappe), and it functions as the direct object of kaufen, meaning it is in the accusative case. The feminine accusative form of this pronoun is welche. If it were replacing a masculine mass noun like der Klebstoff (glue), it would be welchen.

Question 3

Could I use etwas instead of welche in this sentence?

Accepted Answer

Yes. For uncountable singular mass nouns like Pappe, you can also use etwas to mean "some" (...sondern müssen auch etwas kaufen). However, welche is very common in everyday speech and explicitly carries the gender of the noun, which can make it clearer what you are referring back to.

Question 4

Why does the verb müssen come immediately after sondern?

Accepted Answer

The word sondern is a coordinating conjunction, which sits in "position zero" and doesn't take up a standard slot in the sentence. Normally, the subject would come next. However, because the subject (wir) is exactly the same in both halves of the nicht nur ... sondern auch structure, it is often dropped in the second half. This leaves the conjugated verb müssen right next to sondern.

Wir brauchen nicht nur Pappe, sondern müssen auch welche kaufen.

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