Confusion over Vowels

I'm confused on when I would use ㅓand 一 I can't differentiate which one would be used in a word.

Hi Cristian, Thanks for your email. Korean vowels can be a bit tricky sometimes! Are you having trouble identifying the vowel when a native speaker is speaking? If so, it's probably just a case of getting used to the sound and having more listening practice. I would recommend reading while listening when possible. Your Interactive Audio lessons have transcripts of the conversations so you can listen and read along. You can also try watching a Korean movie with Korean subtitles which will help you connect the spoken word with the written word. I hope that helps!