Hi everyone! I'm currently at Yonsei KLI in Level 1 and we just learned the different ways to give a reason to a statement/statement but i cant seem to understand in which situations i should use which of the three options. I know i cant use 아서/어서/여서 for suggestions or demands with f.e -십시오.

Whenever my classmate and i asked our teacher, she always said that it has to do with how much background/context the listener already knows, but it just confuses me and i cant get my mind around it. I even asked my situationship who is fully korean if he could explain it but he just said that "i just know it". 🥲

Im aware that in casual conversation it might not matter which one you use but i need to know the difference and be able to use them correctly for my finals. Please if someone can help i'd greatly appreciate it!

I’m curious about when it sounds natural to use 이다/아니다 with 겠다. For example, when making a guess:

- 그건 여권이 아니겠어요 instead of 그건 여권이 아닐 거예요

- 그 사람이 의사(이)겠어요 instead of 그 사람이 의사일 거예요

I don’t see 이다 used this way, so I’m wondering in what kinds of situations this combination would actually sound natural.