in the afternoon泛指“下午”，如：They have sold up ten crates of mineral water in the afternoon.on the afternoon一般指具体某一天的下午，如：on the afternoon of June 5th.
in the afternoon例句
In the afternoon they went shopping.
他们下午去购物了。
Outbreaks of rain are expected in the afternoon.
下午将有暴雨。
We got to the beach at five of one in the afternoon.
我们下午1点差5分的时候到达海滩。
on the afternoon例句
Where were you on the afternoon of May 21?
5月21日下午你在哪里？
You were pulled over on the afternoon of May 19?
你在5月19日下午被警方拦停？