Complex sentence: During the next week, on Benjamin's 21st birthday, he walks around, thinking about his life, and when he got back, his mother suggest that Elaine wouldn't return on Saturday because she has to stay up in Berkeley for summer school.
Split into simpler sentences: During the next week, on Benjamin's 21st birthday, he walks around, thinking about his life. When he returns, his mother mentions that Elaine wouldn't return on a Saturday because she has to stay up in Berkeley for summer school.