失去后才明白他早已经不在
[shī qù hòu cái míng bái tā zăo yĭ jīng bù zài]
It means 'Realizing He Was Already Gone after Losing Him', a sad and retrospective expression indicating the protagonist's realization of the loss of an important person in his/her life. This often implies regret for not treasuring him enough when he was around.