It seems to me that the pronunciation of ㅓby the speaker can be hard to distinguish from ㅏ. Just by listening to the speaker, syllables containing ㅓsound a lot like "Ah" a lot of the time. Is this common in Korean?
It makes sense to me that you confuse these two sounds. When you first learn a new language, initially your brain tries to fit the sounds into the existing categories that you use for understanding your own native language, or possibly, other languages whose phonology / soundscape you have become more familiar with.
The sound ㅏ is very similar to the "ah" sound in English, similar enough that your brain automatically hears this sound as "ah" without much variation.
ㅓ, on the other hand, is a different beast. It's kind of "in between" sounds that we have in standard American English. I think of this sounds as being somewhere between an "ah", the short "o" sound as in "hog" or the "aw" in "saw", and perhaps a tiny bit like a longer "o" sound too, and perhaps even a short "uh" sound. Keep in mind, in English, the "ah" and "o/aw" sounds are already pretty close to each other...and the Korean ㅓ sound is often just a little closer to the "ah" sound.
So...when you hear this sound, your brain often tries to map it onto the English sounds, and, depending on the context, the person speaking it (there's a range of pronunciation of sounds in any language!), and what you're used to hearing (different regions / dialects of English pronounce sounds differently so what you're surrounded by affects how you process sounds), your brain is going to put it in one category or another.
And it makes sense that it would get thrown into the "ah" category, to me, because it's already kinda close. This same phenomenon plays out in other languages, like native Spanish speakers having trouble distinguishing the long and short "i" sounds in English, because that distinction doesn't exist in Spanish.
The sounds are not the same, and Koreans generally distinguish them. You just need to keep listening closely. Listen, try speaking the sound out loud. Listen to how different native speakers speak it and try to hear the difference.
With time and practice it will sound different to you.
ㅓ is more like "uh" (as in "uhhh, what did you just say?)
Whereas ㅏ is like "ah" (as in, ahhh okay I get it now)
Pretty much, though I'd say that ㅓ would fit more with "uh" in Received Pronunciation (British). But even then it's not that back into your mouth. It's more like between the middle of the roof of your mouth and your uvula. (Can't say exactly, I'm a newbie when it comes to mouth positioning for sounds)
The mechanized pronunciation in the early sections of the course, unfortunately, leaves a lot to be desired. The difference between "아" and "어" is really quite distinct in normal speech. The '아" pronunciation requires opening the mouth much wider. I have much more difficulty distinguishing the "어" from "오", which is reflected in my poor Korean spelling. I highly recommend supplementing the early units of the course with any of a number of good Youtube courses to gain a better idea of the natural pronunciation of each letter and each letter combination. There are a number of threads in the Korean Forum with suggested sites.
I agree with you - Duolingo's Korean course is in a very early stage of testing. I do not recommend it for beginners!
If you follow American pronunciation, this is how they sound:
어 vowel sound - cough, talk, walk, saw
아 vowel sound - lot, cot, rot, prop
I find it fascinating how much variation there is in English, even variation that we don't normally notice. For example, the other day I was with friends and some of us pronounce "Mary", "merry", and "marry" exactly the same way, and some people pronounce all three differently, and some people lump two together but pronounce one differently.
Merry and Marry are totally different for me. And I know Mary should have a long A, going by English spelling, but I always pronounce how people tell me their names are pronounced. And I usually hear Mary=Marry, M-a-ry (stress on a, and a pronounced as in father, Like Maria without the final a)
They are very similar. 아 is a mid level vowel that resonates at the front of the mouth and 어 is a mid level vowel that resonates in the back of the mouth at the precipice of the esophagus. That is how is differentiate them.
As with any foreign language, it is simply a matter of exposure and training the ear. As you listen to the language more and more, you will be better able to detect and distinguish the sounds.