BEFORE是介词吗?AFTER是介词呀.BEFORE呢?I go to school after he playing...在他打篮球之后.在他打篮球之前.翻译这两个句子.对了.是介词后的动词+ING还是紧跟在介词后的动词+ING还是介词后的第一个东西加ING?

应该是 conj. 连词～
在…以后
After he goes, we shall eat.
他走之后, 我们就吃饭。
介词的话就是
After the play they called for the author to show himself.
剧终之后他们请求编剧和大家见面。
介词后是doing～～
After having listened the doctor's words, Xiao Ming nodded and satisfied···当然是动词了～～having done

对.
在他打篮球之后。
After his playing basketball.
在他打篮球之前。
In his play basketball before.

既可以当介词也可以当连词的，after 和before都是这样

before也是介词

in,on,out,by,inside,outside,beneath,off
down ,up,behind,under.
这些是介词,既然ON,IN是,BEFORE也是

after before 介词
介词后加ing 是相当于名词的词 .