Complex sentence: John Mark "Johnny" Galecki (born April 30,1975) is an American actor best known for his role as David Healy in "Roseanne" from 1992--97 and as Dr. Leonard Hofstadter in "The Big Bang Theory" since 2007.
Split into simpler sentences: John Mark "Johnny" Galecki (born April 30,1975) is an American actor. He is known for his role as David Healy in "Roseanne" from 1992--97 and as Dr. Leonard Hofstadter in "The Big Bang Theory" since 2007.