Learn Hardcore German: Dieses Buch ist interessant, aber das andere ist langweilig. - This book is interesting, but the other one is boring.

Because Buch is a neuter noun (das Buch) and it’s the subject (nominative case) of the sentence. The demonstrative pronoun dies‑ must be declined for gender, number, and case. In nominative singular neuter, the strong declension of dies‑ is dieses, so we say dieses Buch.

What case is dieses Buch in, and how do you know?

It’s in the nominative case, which marks the subject of a clause. Here, dieses Buch is what “is interesting,” so it must be nominative.

Why does andere have an -e ending in das andere?

In das andere, das is acting as a definite pronoun (“the other one”), and andere follows it. That combination triggers the weak adjective declension, where nominative singular neuter endings are -e. Hence das andere.

Is das in das andere an article or a pronoun?

Here it’s a pronoun, standing in for das andere Buch (“the other book”). The noun Buch is clear from context, so it’s dropped.

Why aren’t interessant and langweilig declined (i.e. why no endings)?

They are predicative adjectives, coming after the linking verb ist. Predicative adjectives in German never take endings—they remain in their base form.

Why is there a comma before aber?

German requires a comma before coordinating conjunctions like aber when they connect two independent clauses. Each clause here has its own verb (ist), so the comma is mandatory.

Why is aber used instead of sondern?

sondern is only used when you correct a negation (e.g. “nicht A, sondern B”). Since the first clause isn’t negated (“Dieses Buch ist interessant”), you use the neutral contrastive conjunction aber.

Does aber change the word order in the second clause?

No. aber is a coordinating conjunction, so it does not send the verb to the end. The normal subject–verb–object order stays in place: aber das andere ist langweilig.