Complex sentence: Soon after she begins receiving more letters and begins to wonder about her husband's mysterious past and why he never mentions anything of his past to her, becoming even more alarmed when she searches a locked room and finds a box of old letters from an ex-lover.
Split into simpler sentences: Soon after she begins receiving more letters and begins to wonder about her husband's mysterious past. She becomes even more alarmed when she searches a locked room and finds a box of old letters from an ex-lover.