Complex sentence: Joe initially hides the postcard from his wife, Ruth, however he soon reveals to her not only its existence but that of a diary Joe kept twenty years before, when Joe and Ruth met Astrid while visiting Denmark.
Split into simpler sentences: Joe initially hides the postcard from his wife, Ruth. However, he soon reveals to her not only its existence but that of a diary Joe kept twenty years before, when Joe and Ruth met Astrid while visiting Denmark.