As an ESL teacher, how would you explain the difference between:
"The stock market is not safe for everyone."
"The stock market is unsafe for everyone."
What I really mean is: It's fairly easy to state the difference. But how do you make it make sense to ESL students? Or, failing that, how do you relate it to other English usages? Or, failing that, how do you present it so they can remember it?
You've stated the difference well. And you've compared it to "dislike" versus "not like," which I think works. Is there any way to make sense of why we know "not safe" means something different from "unsafe" in this context and why we know "dislike" can mean something different from "not like"? I'm looking for a rule or at least an identifying term that an ESL student may use to generalize to other words or word pairs in English?
The reason I seek a rule or at least an identifying term is that in many situations these words mean the same. In technically-correct English, "I dislike chocolate" does mean something different from "I do not like chocolate," but in colloquial English their meaning is the same. Also, "It is not safe to jump from the roof" has the same meaning as "It is unsafe to jump from the roof."
Yet when we use them in another sense, they mean something quite different: "The stock market is not safe for everyone" versus "The stock market is unsafe for everyone." Is there no rule, or at least a name for these different usages that will enable us to talk about them generally?
I don't think you will find any rule to cover what we are talking about. As we know, its possible to say, "I don't like Fred, but I don't dislike him; at the same time, it's not just in colloquial English that"I don't like chocolate" means effectively the same as "I dislike chocolate" - though "I dislike chocolate" often has a stronger feel.
We are into the area of semantics here, where PhDs can be got for looking into such things.
I think you are on thin ice when you talk about "technically correct English". This often means "what some writer(s) have decreed that these words/expressions must logically mean". Language does not fit into convenient patterns and slots.
Our learners, and many of us teachers, need to realise that words have meaning only in context, and that groups of words - phrases, sentences, whole chunks of sentences, also have meaning only in a wider context.
This makes teaching, and learning, rather more difficult than it perhaps was fifty years ago. On the other hand, at least most students today don't learn a language that was spoken, on fairly formal occasions, by only a minority of native speakers.
Perhaps we should all pay less attention to 'technically correct English' and try to deal with English as she is spoke.
Sorry. A hobby-horse of mine.
Your point about whatever is meant by "technically-correct English" is well taken. What I really meant was that technically "I don't like chocolate" could mean "I don't like it, but I don't dislike it, either"–just as you said about your bland friend, Fred.
As you point out, context is all important. One of my French instructors often was reduced to shrugging his shoulders while explaining, "C'est franšais!" More and more I'm thinking that this falls into that same category: It's just English.
(Of course, I'm addressing American English only here. I wouldn't presume that this discussion necessarily applies across the board. I mean, when I consider the effect on a sentence of the different treatment of collective nouns in American versus British English, . . . . )