These explanations are simplified from Grevisse’s Le Bon Usage, 14th ed. §§1010-1026
Historically, ne was the only expression of the negation, it functioned somewhat like not in that regard. With time, what were originally emphatic (pas being the most common) or additional adverbs (rien, jamais) etc. came to become required, leading to ne+pas or another adverb being the normal expression of negation (ironically in modern colloquial French, ne is now dropped instead).
The original usage remains generally possible, however, as you note in your last example (and pouvoir is in fact one of the verbs that most naturally allows the construction), but is generally associated with a rather literary or elevated style, or with a limited number of expressions where it is obligatory, such as n’importe qui, Il ne boit ni ne mange, je ne sais que faire etc.
Besides the complex issue of what are known as ne explétifs (§§1023-4), where the ne has little to no semantic connection to negation, Grevisse (§1018) freely admits that ne que is an oddity. But it is one with a prestigious pedigree, since it traces right back to the Latin non aliud quam, which actually expressed that the two items thus connected were not different, hence, the same. This eventually evolved (though Grevisse doesn’t detail the semantic evolution) into a sort of “negated negative”, hence “only”.
Ne as a part of double negation:
- ne … pas
- ne … aucun
- ne … jamais
- ne … plus
- ne … rien
- ne … personne
These ways of negation are often mutually exclusive, for example:
Ne … pas cannot be used with aucun, jamais, or personne.
- Wrong: Je n’ai pas aucun ami.
- Right: Je n’ai aucun ami. I have no friends.
- ne … que means “only”
- ne … pas que means “not only”
- Je ne regrette qu’une chose. I regret only one thing.
- Je ne regrette pas qu’une chose. I don’t regret only one thing.
- ne in formal French:
Cesser, oser, and pouvoir don’t need pas:
- Elle ne peut venir avec nous. She can’t come with us.
“On ne peut avoir de dépendance” is exactly this case.
Source of examples:
I once stumbled upon the wiki article for Jespersen’s Cycle which immediately helped me really grok the negation (and confirm my suspicions that ne is the “true” particle for negation).
About ne que: I know read it as “nothing but”. Actually, as I’m German I’m reading it as “nichts als” which is how Germans say “nothing but” – that works quite well because “als” literally means “than” in English or “que” in French and is used as the particle in comparative. So that’s how ne que means “only” – maybe no inherit explanation, but it’s actually not that unfamiliar.
In the same vein, ne … aucun, ne … plus and almost all other negations make a lot more sense.