Are "att rita" and "att teckna" exactly synonyms or is there some difference in use between the two terms?
They're close, but not 100% the same. rita is more often used about children drawing, teckna is often used for more complex drawings. rita is used for drawing maps and diagrams, but teckna is for more artistic drawings.
Thank you very much for the detailed answer, now I understand. Thanks also for the answer in the other thread.
Jag rita en äpple is wrong?
Two errors: rita is the infinitive, but you need the present ritar. And äpple is neuter, so it's ett äpple.