Complex sentence: The Serbian Progressive Party was formed when a group of 21 MPs led by Tomislav Nikolić of the Serbian Radical Party (SRS) disenchanted with the direction of the party, left and formed the "Forward Serbia" parliamentary group.
Split into simpler sentences: The Serbian Progressive Party was formed by a group of 21 former Serbian Radical Party (SRS) MPs led by Tomislav Nikolić. Disenchanted with the direction of the party, the members left and formed the "Forward Serbia" parliamentary group.