He had to drop out of the race because his leg was injured.

因为他的脚受伤了，他不得不退出比赛。

drop out有退出和退学的意思。

He dropped out of school when he was fifteen.

他15岁时退学了。

如果drop out连成一个单词，写成dropout或drop-out，就变成一个名词，意思是退学生。

His mother doesn't want him to be friends with those dropouts.

他的妈妈不想他和那些退学生交朋友。