Complex sentence: The Soviet tactical aim was the defeat of the German 18th Army, the capture of Tartu, reaching the coast of the Bay of Riga and cutting off the Army Group Narwa in Estonia.
Split into simpler sentences: The Soviet tactical aim was to defeat the German 18th Army, capture the city of Tartu. From then on the Soviet command planned to reach the coast of the Bay of Riga and cut off the Army Group Narwa in Estonia.