When spelling out a word, how can the speaker emphasize a certain vowel? In English this can be done, for example, by used the word apple when emphasizing an a; how about in Korean?
Korean really has no emphasis on vowels. I think the mean by "emphasized vowels" are actually one of the consonants like ㅃ,ㅉ,ㄸ,ㄲ,ㅆ. But these are consonants not vowels.
I really do not know what kind of vowels of Korean have emphasis. Unlike English, you really do not have to give emphasis on Korean.
If you meant by grammatically, then I really need some time to get clear and straight answer for it.
If you meant by pronunciation, then I really need to record my voice or make a video explaining it.
The parts you are asking is quite complex. Please be more specific on what you are asking.
The difference between 오 and 어, is that the first is a short " o" sound like in while the latter is more of a prolongued o like "ooh" from pronouncing the english alphabet but with a little softer intonation. also the first is more pronounced with your throath whith an open mouth. the latter is less created with your throath and requires an o shaped mouth.
similar is with the 요 and 여 sound its the similar sound with an added y. so yo and yoh. remember that the latter sound more like the english expression "yo" with a softer intonation on the "o" that in the english expression, the korean "yo" is a short and hard "o"
I unfortunatly counld not come up with an english wordt that properly pronounces this hard and short korean 오.
I hope this explanation somewhat helps, I myself also still have problems with proper pronounciation during pronounciation class. but for me the ㅍ ㅂ ㅃ, ㅅㅆ and ㄱ,ㅋ,ㄲ. are a real trouble because I have a hard time hearing the difference when koreans pronounce them. let stand pronounce them myself.
since it sounds like your doing selfstudy let me hint u on one weird pronounciation rule. then a ㅅ and l are combined like the word 신문(newspaper) is not pronounces as sinmun/sinmoen but as shinmun/shinmoen. while with other combinations of vowels and ㅅ the h is usually not there (correct me if I am wrong)
Are you talking about when people ask for spelling clarification like "is that 'a' as in apple or 'e' as in egg?" I don't know if it's done for the basic vowels like ㅏ,ㅑ, etc, but for distinguishing between 레 and 래 you can say like 'uh ee' or 'ah ee'.
If you want to differentiate ㅕ vs ㅛ, keep in mind that we never use vowel alone but it is always accompanied by ㅇ.
So you are actually comparing 여 vs 요.
You can use words that contain either 여 or 요 to distinguish them.
Example) 여자할때 여, 요술할때 요