I can imagine that it is hard to see the different between does two. But herhalen means repeat, you repeat a exercise or something you said. (voorbeeld) Ik herhaal deze oefening omdat ik een fout heb gemaakt. (I repeat this exercise, because I maid a mistake) Nazeggen means that someone, a teacher, says a word and you repeat it exactly as she/he says it. So for example a teacher gives a dictation. She says a word a you repeat i correctly. Ik wil dat je mij nazegt om zo te zien of je het goed uitdrukt. (I want you to repeat after me, so I will be able to tell if you pronounce it right.) I hope it helps, good luck with Dutch ^^

i don't know if "nazeggen" even has a word in english. At least i don't know it. But herhalen just means repeat. And nazeggen means that you repeat what somebody just sad. So nazeggen is always about talking. Where herhalen can be about just anything!