Could you please explain, simply and clearly, how the pronunciation of '어' and '으' differs.

Great question — this is the pair that trips up almost every English speaker, so you are not alone. The short version: 어 is an OPEN mouth, 으 is a NEARLY CLOSED mouth. ■ 어 — drop your jaw Say the English word "sun" and hold the vowel: s-uh-n. That "uh" is very close to 어. Your mouth is open, your jaw is down, your lips are relaxed and NOT rounded. 어디 (eodi) — where 엄마 (eomma) — mom 서울 (Seoul) — Seoul ■ 으 — almost close your mouth and smile Now bring your teeth close together, almost touching. Pull your lips wide and flat, like a small stretched smile. Make a sound in that position. That is 으. There is no English vowel that does this, which is exactly why it feels hard. That is normal — it is a new sound, not a sound you are getting wrong. 음식 (eumsik) — food 그 (geu) — that 스무 (seumu) — twenty ■ The quickest way to feel the difference Put a finger on your chin and say 어 — 으 — 어 — 으. 어 your jaw DROPS 으 your jaw barely moves If your chin is moving the same amount for both, one of them is wrong. ■ One pair to practice These two words differ ONLY in this vowel, and you will use both every day: 그 (geu) that 거 (geo) thing Try: 그거 (geugeo) — "that thing". You can feel your jaw drop on the second syllable. ■ A shortcut you already know Korean borrows a lot of English words, and it uses 으 to fill the gaps between consonants: 스트레스 seu-teu-re-seu stress 크리스마스 keu-ri-seu-ma-seu Christmas Every one of those 스/트/크 syllables is the 으 sound. So you have already heard it many times — you just have not had a name for it. ■ If you want to hear them The lessons have audio for both: 어 → https://kongle.app/learn/basic-vowels-1/ 으 → https://kongle.app/learn/basic-vowels-2/ Play them a few times back to back with your finger on your chin.