Thanks so much for your comment, Bingley... Let’s see... I see why you say that "I never used to play in the park" is a negative sentence, as it is definitely one in terms of meaning (something was NOT so). However, in terms of grammatical structure, it is no different from “I always used to play in the park”, because the verb form is not in the negative. It is this that I was referring to earlier: when you make a negative form of a verb, it requires an auxiliary “do” plus a negative particle “not” and then the base form of the lexical verb. So in fact I was very wrong in citing this as an example of “negative” sentence, because I was really only considering the negative structure of the verb phrase itself. I see that now.
However, the point in question was “didn’t use to” and “did / didn’t [noun / pronoun] use to…?”. As you say, “I didn’t use to watch TV so much” is less frequent than “I never used to watch TV so much”, and I’d suggest that this is because the first is actually not so correct. As I said before, what we learn as EFL students is that “used to” can only be used in affirmative structures in the past tense. I am adventuring here, but perhaps native speakers are reluctant to use the negative and interrogative structures of this verb because they are consciously or unconsciously aware of this norm?
(I will say, though, that while “I didn’t use to go to the park” and “Did you use to go to the park?” sound wrong to me, “Didn’t you use to go to the park?” sounds not-so-wrong. I don’t really know why…)
