Is 'Three years after the attack, Betty Elliot was living among the Aucas.' a complex sentence or a simple sentence?
As written, this is a simple sentence because it doesn't contain any dependent clauses.
Examples of a complex sentence are as follows.
Because my coffee was too cold, I heated it in the microwave.
Though he was very rich, he was still very unhappy.
She returned the computer after she noticed it was damaged.
When the cost goes up, customers buy less clothing.