成绩下降用英语可以表达为"decline in performance"。这个短语的来源可能源于拉丁语"declinere"，意思是"放弃"或"退步"。

例句：

His grades have been declining since he started playing video games all day. (他自从整天玩电子游戏后，成绩就一直在下降。)

She was devastated by the decline in her performance on the final exam. (她对期末考试成绩的下降感到非常沮丧。)