Complex sentence: The Queen consorts of Denmark held the title of Countess (later Duchess) of Oldenburg, but it wasn't until 1667 to 1773 that Denmark control Oldenburg, then it was passed to the Russians, who were rule by another scion of the House of Oldenburg.
Split into simpler sentences: The Queen consorts of Denmark held the title of Countess (later Duchess) of Oldenburg, but it wasn't until 1667 to 1773 that Denmark controlled Oldenburg. Then it was passed to the Russians, who were ruled by another scion of the House of Oldenburg.