Related: Past Subjunctive - dürfte + past participle + haben vs. hätte + infinitive + dürfen. Somewhat less related: Konjunktiv II with modal verbs
I know that translating German modal verbs to English usually requires some rephrasing when they are negated. For example Wir müssen nicht means "We don't have to" rather than "We must not." I'm wondering if the same thing holds for the subjunctive/''Konjunktiv'', and if there are any subjunctive modals, with or without negation, which have unexpected meanings.
For example, in the German and English versions of the same game I ran across:
''Der Junge hätte nicht mit uns reden dürfen, das ist gegen das Gesetz.''
"That boy should not have spoken to us, it's against tribal law."
The English is a bit ambiguous here, but in context I think it means "That boy was not supposed/allowed to speak to us,..." Translating this into German, I would have gone with ''Der Junge durfte nicht mit uns sprechen, ...'' which uses the indicative. So I'm not sure what the difference is here between the indicative and the subjunctive. Translating the German into (rather awkward) English, I get "The boy would not have been allowed to talk to us,...".
The answer to the first question linked above says something about ''dürfen'' in the past subjunctive having a special meaning. I gather this is referring to meaning 2 in DWDS where it says it can mean something will probably happen. I don't think this is applicable to the previous example but it is another case where a subjunctive modal verb has a surprising meaning. DWDS also lists such a special meaning for ''müssen''. Are there any others I should know about?