。
She was sitting on the sofa reading a magazine.
Literal
She [topic-は] sofa [on-に] sit [then-て] magazine [object-を] was-reading.
A textbook depiction of overlapping continuous action: te-form chaining links 座って (sat down) to 読んでいた (was reading) so the sequence reads 'sat down and was then reading.' English collapses these into a single image — 'sitting...reading' — but Japanese keeps them as discrete, ordered events linked by て. に here marks the destination of 座る; if it had been で, the focus would shift to the sofa as the location of the reading rather than the seat itself.