Complex sentence: In the 17th century Milton kept the sonnet form alive while also expanding the subject matter--which hitherto had mainly been love--by writing about ideas, events, history, and contemporary issues.
Split into simpler sentences: In the 17th century Milton kept the sonnet form alive while also taking liberty with it. He expanded the subject matter of the sonnet away from just the typical love poem by writing about ideas, events, history, and contemporary issues.