"In the afternoon"
You could say 'om eftermiddagen', about actions always taking place in the afternoon
Maybe it's used like that in some dialects, but it's too dialectal to be an accepted answer. You could easily be misunderstood if you used that instead of på eftermiddagen.
On the other hand om eftermiddagarna works in Standard Swedish for talking about things you do every afternoon, but that would be 'in the afternoons' in English.
can someone please explain why: på eftermiddagen = in the afternoon i eftermiddagen = this afternoon ?
Different languages use different prepositions to express things. So you just have to learn that på eftermiddagen corresponds to in the afternoon in English and i eftermiddags (note the correction) corresponds to this afternoon. Another example is i två år which is for two years.
Yes, sorry I misunderstood you then since ”this afternoon” can also be past tense. Then you would want i eftermiddag to mean the future.