Non native speaker here, it's the second time this week[1] that I see "didn't used to..." instead of what I believe to be the correct form "didn't use to...".
Aren't verbs after "did/didn't" always supposed to be in the present tense? For example you would say "I didn't see you last night" rather than "I didn't saw you last night". Is there a grammatical rule I'm missing?
Over at the English Language Learners [0] Stackexchange, reading through the answers and how they describe their references (I didn't click through to read those references), it seems like "didn't used to" is more acceptable in general for both American and British English, even though it shouldn't be correct because it breaks the rules.
I like this last part of one of the answers in particular:
> One of the strongest arguments against treating used to as a standard verb form is the pronunciation. According to the Cambridge Dictionary, with the verb form of use, the s is voiced /juːz/ whereas in the noun form it is not voiced /juːs/. In I used to and I didn't used to, the s is unvoiced. It's definitely not a noun, but it is also not a standard verb form: it is something unique, and that's what upsets the grammarians.
Aside, like sibling comment, I'm also a native American English speaker and it's always been "didn't used to".
I'm also a non-native English speaker, I also found the the title of the linked article weird and also saw the title of the Vsauce video recently. This all made me confused and I'm happy that you brought it up here and that people chime in to clear the situation up.
As a native UK speaker, for me it has always been "used to" and "didn't use to", although up to this moment I have never noticed the subtle difference.
Aren't verbs after "did/didn't" always supposed to be in the present tense? For example you would say "I didn't see you last night" rather than "I didn't saw you last night". Is there a grammatical rule I'm missing?
[1] the other one was this video by youtuber Vsauce, https://www.youtube.com/watch?v=vjqt8T3tJIE
EDIT: wow so many interesting responses, thank you!