But you see that is the point, what one person may categorize as fiction, others may categorize as verifiable fact. That is the whole crux of the issue which you continue to miss completely.

Provided we are speaking english, and we both agree that fiction means something that does not correlate to observable peer reviewed reality, and that verifiable fact means an objective thing that can be observed, tested, and verified by multiple individuals, you can certainly say that someone is wrong or right.

Whether or not you classify it as a fact is irrelevant. That's your opinion. And has no bearing on the actual reality of things.

language is subjective. We can sit here and fight over definitions all day and never get to a consensus, because it's subjective and entirely an opinion.

Of course it is an opinion. In fact, it is my opinion that it is an opinion :)

Well no. Excusing the subjectivity of language (it can be your opinion that what is currently happening shouldn't be classified as 'raining'), the fact of whether or not water is falling from the sky is not an opinion. It's objectively verifiable and testable. You may have opinions on what we classify this as, but that's a language game. Your opinion isn't about whether or not water is falling from the sky, but rather whether or not we should call it 'rain'.

Also, I think you may be defining opinion different than almost everyone else, myself included.

An opinion is an untestable subjective claim based on what 'should' be.

The generally accepted definition is that opinions are beliefs or judgements about observable reality.

This is a nonsensical definition. You are trying to claim opinions are true facts about reality. Which is nonsense. "The earth revolves around the sun" is not an opinion. "Rain falls from the sky" is not an opinion. Yet, both of these are beliefs/judgements about observable reality. "This website is hubski" is another one. It's not an opinion.

And yes, such beliefs or judgements can be wrong.

That's because beliefs/judgements aren't opinions. Beliefs are incorrectly accepted facts. Judgements are just assertions.

That is, how does one differentiate between 'knowing a thing to be true' and 'believing that a thing is true'.

The former is certain, the latter is mistakenly certain.