"It was the biggest stress in my life."
Translation:To był największy stres w moim życiu.
Yes, it serves as the subject of the sentence here, exactly like "It" in the English sentence.
I thought subjects were more optional in Polish, like in many other languages (Spanish, for instance). I also thought it was already implied by the inflection of był.
Yes, but third person pronouns are omitted less often. If you omitted "to", this sentence wouldn't describe anything. It would mean that "There was the biggest stress in my life".
Ah, I see the difference now. I'd forgotten forms of być can also mean there is/was/etc. That makes complete sense. Thanks!
If the subject here was on/ona/ono, it probably could be omitted. But this sentence could be also translated as "This was the biggest stress in my life", and it's hard to imagine losing "this" because there really won't be any subject, not even the implied one.