I find the former (we used not to be allowed to drink coffee) sounds contrived although it does comply with prescriptive grammar. It is negated, past imperfect, passive voice. As such it cannot be applied to a isolated event.
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WanwoYou might find the first in an older English book, or used by people who want to sound 'sophisticated' but the second sentence is 10 times clearer. Clarity of expression should be one of your highest priorities.Again, I was asking about the aspectual differences between those sentences.
MilkyI was asking about the difference between the two regarding Aspect.I thought I'd answered that above. "Used to" is imperfect, or if you want to use the word aspect, http://en.wikipedia.org/wiki/Imperfective_aspect, used in this case to denote an ongo