Complex sentence: Impossibilism insists that socialists should focus only or primarily on structural changes (or revolutionary changes) to society as opposed to implementing social reforms, and argue that revolutionary political action is the only viable method of instituting the structural changes necessary to bring about socialism.
Split into simpler sentences: Impossibilism insists that socialists should focus only, or at least primarily, on structural changes (or revolutionary changes) to society as opposed to implementing social reforms. They argue that revolutionary political action is the only viable method of instituting the structural changes necessary to bring about socialism.