蓦然回首已是物是人非
[mò rán huí shŏu yĭ shì wù shì rén fēi]
When I suddenly look back everything stays but people have changed illustrates a sentiment reflecting on past times after a period. One realizes that while things haven't altered significantly over the years, significant individuals involved have matured, departed, etc. Emphasizes change, nostalgia for the passage of time, the transience of people in life and the helplessness caused by inevitable shifts.