This might be true regarding to homonyms, but at least the other way round it works: Once you’ve understood French pronunciation and spelling rules, you can properly pronounce any French word you see in written form, even if you’ve never encountered it before. There are exceptions, the most common probably being femme, but much easier than in English where I’m sure to have pronunciation problems when I see a word for the first time; and even if I already know the term, I sometimes have to browse my memory for the correct pronunciation rather than to say it intuitively. It took me quite a while, for example, to learn how to stress advertisement correctly, although it’s not that rare a word and I had used it (incorrectly) many times before. Or look at the various ways to pronounce the vowel u in “butter,” “church,” and “cute.” You simply have to learn each word’s pronunciation individually.
I’m missing how that relates to what I said; Bill H. didn’t mention anything about physics or the imperial system.
In every case you cite the word “not” is one of the negatives, while the other negative is a word with a negative affix (such as “im-” or “un-”). While it is useful to cite several forms to show that the phenomenon is not idiomatic but applies in several situations, you have really only given one context in which two negatives equal a positive. Your “can’t not” type example also uses not and a negative affix, this time the suffx “-n’t” (which is not quite the same as its full form “not”).
In the cases you have cited the common feature is that a negative (not) is modifying a word that happens to have a negative affix on it. Two different levels of processing are involved here, the syntactic and the morphological.
As an example we take your sentence fragment “it’s not impossible to…”. The word “impossible” is made up of the morphemes “im-” and “-possible”. When you put them together they form a word that takes the meaning of possible and negates it. That word, “impossible”, is then modified by a separate word, the negative “not”. The “not” does not modify both morphemes “im-” and “-possible” separately, it must act on the word created by the two morphemes (i.e. "impossible). (Using a math metaphor, x(y+z) does not equal x(y) + x(z) where x=not, y=im-, and z=-possible.)
As an example of a double negative equalling a positive there is the sentence: “He’s not going to win no beauty pageants.”
Now, I can understand how this sentence (and probably most other’s like it) can be ambiguous. I would say that the most natural interpretation would be that he won’t win beauty pageants, but the opposite interpretation is possible. Said with normal intonation, I would say that most people would really have to work to come up with the interpretation “he is going to win some beauty pagaents” (I don’t mean that this interpretation will not occur to most educated English speakers, just that the interpretation is one that they will have to work for in their head, if you get my meaning). If one were to give the same words a different intonation then the natural interpretation would move towards the double negative = positive interpretation.
E.g. “He’s not going to win no beauty pageants”. The differing stress pattern is a way of showing differing internal structure.
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