I want to know in what circumstances, I will use these forms
I was taught:
- If I was - if the possibliity exists e.g. if I was a teacher (I'm not a teacher. But I could be one.)
- If I were - if it's no real possibility e.g. if I were you (I'm not you. And I can impossibly be you.)
But I've met several times the opinion the two would be exchangeable. So, it can be that the strict distinction either gets generally weaker or has always been weak or has even never existed in some areas.
- If I was to - if the possibility exists e.g. if I was to teach children
- If I were to - if it's no real possibility e.g. if I were to save the world
'If I was to' and 'If I were to' tell about expectations. I am expected to do something. In contrast, 'I have to' tells about obligation. I am obliged to do something. If an obligation exists it obviously refers to a real possiblity. That's why there is no distinction of 'really possible' and 'not really'. But there is a distinciton of let's say 'condition' and 'possibility':
- If I have to - condition e.g. If I have to do it I (certainly) will.
- If I had to - possibility e.g. If I had to do it I (probably) would.