Yes, you may likey hear koreans pronouce words that start with 'n' as 'd' This is simply because the consonants: 'n' and 'd' sound very similar to Koreans. Example: "Naega" can often be pronounced as "Daega"
This is the same with 'b' and 'm'
Example: Mwo (meaning "what") can sometimes be pronounced as 'bwo'
Same with L and R Example : "Rebekah" can be proncounced as "Lebekah"
Duolingo isn't a reliable teacher, I've recently found. I have learnt more off of youtube videos and free PDF files online in a matter of a few hours vs. a few days. I use this app as a way to just practice now. A good youtube channel to go to is Learn Korean With Korean Class101.com ( thats literally the name, but it's good I swear) and the best thing to start off on is the Korean Alphabet (Hangul). They have a playlist, you should be able to find it easily. A decent website to go to is HowToStudyKorean.com. They aren't exactly the best, but paired with a series of youtube videos it really does make it easier. Hope this helps! :)
A good way to learn I find is copying everything down. I’ve been copying and pasting everything taught onto the wiki: https://duolingo.fandom.com/wiki/Korean_Skill:Alphabet_2
Writing down all the words and letters taught in a notebook/pad has also been very useful too.
I hope that helps. :-)
Have in your mind that korean is a complete different language from english. It's totally normal and commom to have huge differences. If you really want to learn, you should stop associating it with english or your native language and just think this as a whole new thing that you will need to learn everything from zero.