Das Haus, das sie gebaut haben, ist modern und groß.

Das Haus, das sie gebaut haben, ist modern und groß.
The house that they have built is modern and big.

Breakdown of Das Haus, das sie gebaut haben, ist modern und groß.

sein
to be
und
and
das Haus
the house
haben
to have
groß
big
sie
they
bauen
to build
das
that
modern
modern
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Questions & Answers about Das Haus, das sie gebaut haben, ist modern und groß.

Why are there commas around das sie gebaut haben?
In German, relative clauses must be set off by commas. These commas mark das sie gebaut haben as extra information describing das Haus, separating it clearly from the main clause ist modern und groß.
What is the role of das in das sie gebaut haben?
Here, das is a relative pronoun referring back to das Haus (neuter singular). In English it corresponds to “that” in “the house that they built.”
How do I know which form of der / die / das to use for a relative pronoun?

You match two things:

  1. The gender and number of the antecedent (das Haus → neuter singular).
  2. The case the pronoun has within the relative clause.
    In sie haben das Haus gebaut, the house is the direct object (accusative). So you choose das (neuter singular accusative).
Could I use welches instead of das here?
Yes. You can say Das Haus, welches sie gebaut haben, … Grammatically it’s correct, but modern German speakers generally prefer the shorter article‐based form (der / die / das) over welcher / welches / welche, which sounds more formal or literary.
Why do haben and gebaut come at the end of the relative clause?
German subordinate clauses—including relative clauses—use verb‐final word order. That means all parts of the verb (here haben + gebaut) move to the end of the clause.
Why are modern and groß not inflected with endings in ist modern und groß?
Those adjectives are used predicatively after the copula ist, so they remain in their base form. Only attributive adjectives (directly before a noun) take case‐ending suffixes.
Why is sie lowercase here? Could it be Sie?
In this sentence sie means “they” (third‐person plural) and is always lowercase unless it starts a sentence. Sie with a capital S is the formal “you” or the polite third person—entirely different pronouns.