Breakdown of Das ist die Agentur, der das neue Projekt gehört.
sein
to be
neu
new
gehören
to belong to
das Projekt
the project
Questions & Answers about Das ist die Agentur, der das neue Projekt gehört.
Why is the relative pronoun der here when Agentur is a feminine noun?
You're right that Agentur is feminine (die Agentur). However, in the relative clause, it acts as the owner because of the verb gehören (to belong to), which always requires the owner to be in the dative case. The feminine dative relative pronoun is der.
How exactly does the verb gehören assign cases here?
The verb gehören works by putting the owned object in the nominative case (as the subject) and the owner in the dative case. In this clause, the subject is das neue Projekt, and the owner is the agency, represented by the dative relative pronoun der.
Why does gehört go at the very end of the sentence?
The part after the comma (der das neue Projekt gehört) is a relative clause. Relative clauses are a type of subordinate clause, which means the conjugated verb always gets pushed to the very end of that clause.
Why is it das neue Projekt and not das neues Projekt?
Since das Projekt is a neuter noun and we are using the definite article (das), the adjective just needs a simple -e ending in the nominative case. Because the article already clearly shows the neuter gender, the adjective doesn't have to.