Essentially:

1914 - Invasions of Galicia and East Prussia

1915 - Gorlice-Tarnow Offensive and the Great Retreat

1916 - Brusilov Offensive and the Romanian Campaign

1917 - February & October Revolutions, the Kerensky Offensive, Battle of Riga, Operation Albion

1918 - Operation Faustschlag and the Treaty of Brest-Litovsk

Further reading:

• [Books about the Eastern Front in WWI] (https://www.reddit.com/r/AskHistorians/comments/384kqh/whats_some_good_reading_on_the_eastern_front/)