Mi lengua materna también es español pero vivo en un país anglo parlante, así que espero esto te sirva: nice es traducido como "simpático", y a veces también como "agradable". Good es "bueno" (para algo, una situación, o una persona), también significa "bien". I'm good = estoy bien.
Thank you! Maybe it's my ears, but I can't hear the difference, certainly not with the "a" (I have to say "with or without accent", though I've taken on board that that's incorrect, because my keyboard doesn't have those symbols) on Duolingo. I'll try and find the Youtube you mention. I appreciate your taking the time to reply.
A is pronounced like in 'father'. It's represented by ɑ in IPA.
Ä is pronounced like in 'fat'. It's represented by æ in IPA. It's pronounced farther forward in the mouth than ɑ.
O is IPA o̞. Some people pronounce the second vowel in 'Cambodia' with this sound. It's pronounced higher in the mouth than ɑ but lower than u, and with rounded lips.
Ö is IPA ø̞. It's pronounced farther forward in the mouth than o̞, higher than æ, but lower than i. It's at the same height as e̞, but with rounded lips.