Breakdown of Der Ast, auf dem wir die Plattform bauen, ist stabil.
sein
to be
wir
we
auf
on / to (down/up to a level)
bauen
to build
Questions & Answers about Der Ast, auf dem wir die Plattform bauen, ist stabil.
Why is the relative pronoun dem used here instead of den?
Dem refers back to der Ast (the branch), which is a masculine noun. The preposition auf is a two-way preposition. Because we are describing where the building is happening (a fixed location answering 'where?'), auf requires the dative case here. The dative masculine relative pronoun is dem.
Why is the main sentence split into two parts?
In German, it is very common to embed a relative clause immediately after the noun it describes. The clause auf dem wir die Plattform bauen gives more information about der Ast, so it interrupts the main sentence. If you remove the part between the commas, you are left with the core sentence: Der Ast ist stabil.
Why does bauen come right before the second comma?
The section between the commas is a relative clause, which functions as a subordinate clause. In German, subordinate clauses always kick their conjugated verb (in this case, bauen) to the very end of that clause.
Could I use an dem instead of auf dem for 'on which'?