Complex sentence: He took over command of the station on November 25,2010, at the start of Expedition 26 which began officially when the spacecraft Soyuz TMA-19 undocked, carrying the previous commander of the station, Douglas H. Wheelock.
Split into simpler sentences: He took over command of the station on November 25,2010, at the start of Expedition 26. That expedition officially began when the spacecraft Soyuz TMA-19 undocked, carrying the previous commander of the station, Douglas H. Wheelock.