Unfortunately most English speakers don't really understand the reasons we use a lot of the weirder cases & tenses. I don't know if the same is true when learning German as a first language, but certainly in English most people just learn past/present/future and that's about it... You don't really get into the nitty gritty around situations like "I had intended to go to the pub later that day", i.e. speaking about the future from the perspective of the past.

Anyway, in the case of was/were I will do my best with an acknowledgement that I am no expert despite being fluent in the damn language.

Was/were are usually singular/plural words that take you into the past continuous rather than simple past - consider "I worked" vs "I was working", "we worked" vs "we were working". The former of each pair implies that the work was a distinct event while the latter implies it was ongoing (I used it again there with "was ongoing").

The "subjunctive mood", mentioned in the title is about hypotheticals, e.g. "If I were you, I would go to the park today", "I wish I were taller", . In the subjunctive the verb remains in its infinitive form, which in this case is "were".

To be completely honest though.... "I wish I were never born" might be grammatically correct, but to my ear it sounds quite old fashioned, like something a Jane Austen character might say. I don't think the majority of people would blink an eye if you said "was".

There's also "had been" and "would have" to consider... "If I had been taller everyone would have thought I was pretty", this is also a hypothetical but honestly I don't know what the case/tense we're using here is... I'll just have to leave you with that :D