the narrator believes that she was once behind the wallpaper herself
Complex sentences contain an independent clause and at least one dependent clause. This dependent clause can not stand alone.
In this way, the following are complex sentences: (the underlined are dependent clauses)
A. <u>Although he denies them </u>, Dr. Vasquez faced several scandals last year.
B. People tend to use too many clauses in sentences <u>that should be simple (dependent clause)</u>.