Complex sentence: The next morning, upon seeing the blood stain on the bedsheet, Phak realizes that he is the first for Dao who is grieving over her lost virginity, something that she has guarded dearly for a long time.
Split into simpler sentences: The next morning, upon seeing the blood stain on the bedsheet, Phak realizes that he is the first for Dao. She grieves over her lost virginity, because it was something that she has guarded dearly for a long time.