in the afternoon泛指“下午”，如：They have sold up ten crates of mineral water in the afternoon.on the afternoon一般指具體某一天的下午，如：on the afternoon of June 5th.

in the afternoon例句

In the afternoon they went shopping.

他們下午去購物了。

Outbreaks of rain are expected in the afternoon.

下午將有暴雨。

We got to the beach at five of one in the afternoon.

我們下午1點差5分的時候到達海灘。

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日下午被警方攔停？