Question

Rewrite as a complex sentence.
The birds returned to their nest before sunset

Answer 1

Before sunset the birds returned to their nests.

Answer 2

Before sunset, the birds returned to their nest.

When you rewrite a sentence into complex sentence, you can find some independent clauses in the complex sentence.

Even here a depended clause is connected with the independent clause, but if you connect them with the depended clauses then it will lack the elements that would make it a complete sentence.

So you must try to make the sentence with some independent clauses to transform them into complex sentence.