fall behind
fail to keep up落后；跟不上John fell behind in his studies and finally had to leave school．约翰学习跟不上，最后只好退学。
To be promoted or to fall behind is not a matter of salary but even more a matter of selfrespect．升职或者落后不仅仅是薪金的问题，更重要的是自尊。
I don't want to fall behind in my studies．在学习上我不想落伍。
Diligence helps one to go forward，where as laziness makes one fall behind．勤奋使人上进，懒惰使人落后。