on the afternoon和in the afternoon都正確。通常說“在下午”但不具體指出日期,應該用in the afternoon.例如,I went to the cinema in the afternoon.如果是說某具體日期的下午就要用on the afternoon.例如,North Korean forces occupied Seoul on the afternoon of June 28.這和說“在某日”要用on是一致的。

in the afternoon和on the afternoon的區別為：指代不同、語法不同、側重點不同。

指代不同

in the afternoon：在下午。

on the afternoon：下午。

語法不同

in the afternoon：一般說“在下午”用介詞in,但當afternoon前有修飾成分如“某日的下午”或“星期×的下午”時用on,但當其前又有時間狀語時, on也可換為in。

on the afternoon：afternoon與this, that, yesterday, tomorrow, every, all等限定詞連用時相當于副詞短語,其前不使用介詞。afternoon用作狀語時，其前也不使用介詞。

側重點不同

in the afternoon泛指“下午”，如：They have sold up ten crates of mineral water in the afternoon。他們一下午就賣完了十箱礦泉水。

on the afternoon：on the afternoon一般指具體某一天的下午，如：on the afternoon of June 5th。6月5日下午。

on the afternoon例句

A jacket was found on the playground on the afternoon of March 1.

3月1日下午在操場上發現了一件夾克。

It rained on and off most of the afternoon.

下午大部分時間都在斷斷續續地下雨。

in the afternoon例句

One cup of coffee in the late afternoon caused him to stay awake almost all night.

下午晚些時候喝了一杯咖啡，導致他幾乎徹夜未眠。

I'll be back at three in the afternoon.

我下午三點回來。

In the afternoon the cows are rounded up for milking.

下午牛被驅攏在一起擠奶。