"He should never have read my diary"
The given translation was "Él nunca debió leer mi diario". If this is correct, how would you express the sentence "He was never allowed to read my diary", the distinction being that the first sentence implies that he did read my diary, whereas the second sentence does not make this assumption?
He should have never read my diary = Él nunca debió haber leído mi diario (notice that the verb is not in present simple in the literal translation although "debió leer" is also accepted in spanish")
He was never allowed to read my diary = Él nunca estuvo autorizado a leer mi diario
"Él nunca fué permitido leer mi diario" doesn't really make sense in spanish, you would have to add other words like this "A él nunca le fué permitido leer mi diario" to keep the meaning of the sentence.
Allowed in spanish as an adjective is best translated as "autorizado", as a verb "permitir" (not the only translation but two that will not mix). This is to avoid confusing situations with a translation in certain situations.
Thanks Eey91. I like your first translation "Él nunca debió haber leído ..." because it looks more like the English and German constructions. But good to know the other possibilities.