unless & if not

Question

When "unless" is followed by sentence of real condition, is can be replaced by "if not"

e.g.

“You'll fail in French unless you work harder.”
“You'll fail in French if you don't work harder.”

But when "unless" is followed by sentence of unreal condition, the meaning of the sentence is contrary to that of a sentence using "if not"

e.g.

“They couldn’t afford to live in such an expensive apartment unless they were rich.”
”They couldn’t afford to live in such an expensive apartment if they were not rich."

“I couldn’t have got to Slough in time unless I had had a helicopter."
"I couldn’t have got to Slough in time if I had not had a helicopter."

I am wondering whether my understanding is right.

Thank you very much!

Answer 1

You're more or less correct as to the differences between the two.

With an indicative apodosis, You will fail in French, unless and if not have the same meaning and connotation.

Whereas with an subjunctive apodosis, They could not afford to live in such an expensive apartment, "Unless" affirms that the claim made in the apodosis is true, "There's no way they can afford to live in there!", with the implication that the claim in the protasis, in this case, they were rich, is false. In the end, this turns into: They don't live in an expensive apartment, because they aren't rich.

"If not", on the other hand, is a counterfactual conditional (in your case, reversed), and implies the exact opposite, that the claim in the protasis is true and the claim in the apodosis is false, i.e. They do live in an expensive apartment, because they are rich. I did make it to Slough on time, because I do have a helicopter.