These two words are often confused. They are not interchangeable (intercambiable).
I will see you afterwards.
“Afterwards” does not have a complement nor object, because in the context you are referring to something already understood (porque esta refiriendo a algo que ya se entiende).
If you said “I will see you after”, you would have made a mistake. “After” needs an object (…necesita un objeto).
I will see you after the opera (te verá después de la ópera). Here “after” has an object, and the sentence is correct.
You will regret it afterwards (después te vas a arrepentir). Here the context is obvious so no object follows “afterwards”.
You could say “you will regret it after the event” (…después del evento) but why use 7 words when you can say the same in 5 words?