Why is "did not" wrong?
makan terus ....
"they eat but you do not " is a correct and more usual English form of this. The second "eat" is implied in English and would never be used a native speaker.
true!! both are correct and should be accepted as correct .
It about learning Indo not english
Did not and do not should both be accepted, the indonesian sentence has no differentiation between the two, so it could mean eother in english
Is it similar to "tapi"?
Yes, tetapi = tapi.
why tetapi and not just tapi? are they interchangeable and just another confusing addition to an already learned word?
Yes. apparently tapi is just a shortened version of it