一般来说是在介词后面直接跟动词ing形式
但是你举的例子应该两种说法都可以,比如说
他做完作业就去打篮球了
这个句子可以有两种翻译
He went to play basketball after finishing his homework
也可以翻译成
He went to play basketball after he finished his homework
也就是说,如果介词后面是接一个句子,就可以有do,did does的形式,但是如果介词后面紧接动词,那这个动词一定要用ing形式了.