It's almost same, but a little different. They are distinguished by tongue's position. But when we talk, it's hard to distinguish.
That's really hard to distinguish between those vowels from pronounciation.
There's 2 ways to see them differently.
1. make sounds first ㅓ then ㅣ and try to put them together quickly
same as ㅕ + ㅣ and ㅑ + ㅣ
also, "ㅓ+ㅣ" makes sound lil bit longer than ㅏ+ㅣ
same as "ㅕ+ㅣ" makes sound lil bit longer than ㅑ+ㅣ
2. depends on words, we can predict which one is which.
게 = crab 예 = yes (formal)
개 = dog 얘 = hey (calling someone)
I am not a professional Korean teacher.
That's just my opinion.