Question

She did not return even at seven in the evening ( change into a complex sentence)??

Answer 1

A complex sentence is a sentence which contains a subordinate
clause/clauses. Complex sentences contain an independent
clause and at least one ( / many ) dependent clause.

An independent clause makes sense on its on too, while a dependent one doesn't.

She did not return even at seven in the evening is written as
Though it was seven in the evening, she did not return. OR She did not return, though it was seven in the evening.
You may opt to write either one.

Here, "She did not return." is the independent clause, while "Though it was seven in the evening" is the dependent clause.

Hope this helps :)