Sally thinks it will rain.The word, "thinks" convinced about half the class that the sentence is an opinion. They were, of course, wrong. It is a fact that Sally thinks it will rain. Just because the statement is about what someone thinks does not automatically mean that the statement is mere opinion. You could ask Sally, "Do you think it will rain?" and she can give you a definitive answer.
I bring this up because the one nugget of that day's lessons that I think everyone took to heart was the clause,
"Your opinion can never be wrong."This is poorly understood to mean, "I can think what I want to think and you can't ever fact-check me because what I think isn't said to be a fact and this is a license to say whatever I want with impunity." I suspect that the people who thought "Sally thinks it will rain" was an opinion are the same who misunderstood the meaning of the wrongness exemption for opinions.
Your opinion cannot contradict fact. For instance, Sally could not say, "I don't think it is raining" during a storm. Rain would be obvious outside, and her denial of that rain would make her either willfully ignorant or delusional. Not every expression is as obvious to fact-check as whether or not rain is falling, of course. This is where value judgments come into play.
A value judgment is a sub-class of opinion that is concerned with interpretation of fact. Suppose Sally said, "This rain is awful!" "Awful" is a value judgment; it is not a term applicable to a factual statement. There may be farmers whose crops have been in need of that rain who welcome it. How "awful" the rain is, then, is subjective to how receptive one might be to the rain. It's okay that Sally dislikes rain, but she should be more careful in how she expresses this. Sally would be more prudent to state, "I think this rain is awful!" Otherwise, she invites dissenters to challenge whether or not the rain is, in fact, awful and the truth is that there are no objective measurements for "awful."
Suppose, though, that Sally wasn't content to simply declare the rain, "awful." Maybe she's hopping mad and goes into a tirade. "This rain is awful! It ruins everything! You can't go outside in this and do anything, and even if you tried to go somewhere indoors, your hair will get messed up and you can't wear suede and umbrellas are hard to close quickly so you can't just get into a car without getting wet."
It sounds like Sally has made a convincing case for condemning the rain as "awful," but look more closely. "It ruins everything?" Of course rain doesn't ruin everything. In addition to the aforementioned crops, rain does not affect indoor activities. Furthermore, there are people who enjoy rain. It may be true that, depending on the severity of the rain that outdoor activities are impractical, but perhaps Sally is merely being hyperbolic. While it is true that your hair is likely to frizz when humidity is high, that rain is not generally suede-friendly and umbrellas don't always close quickly, is any of that compelling evidence that rain is, "awful?" Maybe you have short hair, aren't wearing suede and you've got a poncho.
Sally's arguments, then, amount to her personal experience with the rain. She's the one who wants to go outside with hair that will frizz while wearing suede. Others will not find her reasons for condemning the rain valid. That the rain is "awful," then, is subject to her values. Sally values dry outdoor activities, frizz-free hair and wearing suede. Others may not. Where things really become frustrating is if we call Sally on how wrong she is, because then she will insist, "It's my opinion and you can't tell me I'm wrong."
Sally is wrong.