Complex sentence: After S.H.I.E.L.D. attempted to replicate Hank's shrinking formula, he resigned and started his own company Pym Technologies, and took in Darren Cross as a protégé, but stopped when Cross became obsessed with learning the secrets of Pym Particles.
Split into simpler sentences: After S.H.I.E.L.D. attempted to replicate Hank's shrinking formula, he resigned and started his own company Pym Technologies and took in Darren Cross as a protégé. Hank Pym stopped his project when Cross became obsessed with learning the secrets of Pym Particles.