Complex sentence: While countercultural undercurrents and subcultures are common, a counterculture is more visible and persists for a significant period of time; when oppositional forces reach critical mass, they can trigger dramatic cultural changes.
Split into simpler sentences: While countercultural undercurrents and subcultures are common, a counterculture includes a significant number of people and persists for a notable amount of time. When oppositional forces reach critical mass, countercultures can trigger dramatic cultural changes.