Not if the verb is "did" - that makes for two past tenses, and that never happens in English. "He used to take the bus" "He did not use to take the bus" NEVER "He did not used to take the bus", because that last version would put the action in the past tense of the past tense.
A better translation: "Why was he not taking the bus?"
you ought never say 'never'. 'did' is, of course perfectly acceptable, even if not used among people whose speech you are familiar with (it's a big world). it's also not your job to mandate how people speak. it's only important that duo recognize acceptable forms. so if duo doesn't accept something you believe to be correct, report it. don't worry about how the rest of us speak.
"used to" is basically an adverb nowadays. Since the d and t sound similar it is mostly pronounced "use to", but could be spelled "use' to" (but no one does).
No one would say "was he not taking the bus" except when they are stressing "not", as in someone was being delinquent for not taking the bus.
It means that he was not in the habit of taking the bus, rather than that he did not take it on one occasion. However I agree that almost nobody would ever say it. I think that this is a case where the Italian has a tense to express the exact meaning (imperfetto) whereas English does not (we have no imperfect tense as such) so we have to either use the simple past tense and let the context make it clear that we are talking a past habit, or we have to use an expression like "use to" or even "would" (why wouldn't he take the bus? [in the past]).
As to whether it should be "used to" or "use to", the simple past is "used to" (eg he used to take the bus) but once you use the auxiliary "did" you have to use the infinitive form of the verb "use". If you don't believe me try "did" with any other verb and you will see it always takes the infinitive not the simple past form of the verb. This is because the "did" is sufficient to indicate the past tense so the past form of the verb itself is not required. Here is a link to a good explanation: http://www.grammar.cl/rules/used-to-use-to.htm
We cannot put such hard and fast rules on what can be said in a language. In fact, it's trivial to come up with an example. Perhaps you're discussing a friend, and how he used to go work, but he always walked. Perhaps person B knows that the bus goes right past the office: "Why didn't he use to take the bus?" or perhaps person B's sentence didn't flow smoothly from brain to mouth, and came out as "Why did he not use to take the bus?"
I think this example is showing the limits of Duo. Italian is a latin language, very opposite to English. This Past Tense lesson is almost illogical , at least for a french like me. The "used to" or "be + ing" isn't natural at all. I will go to the end of the lesson but honestly, it's kinda useless, it will confuse me more.
CChat is correct. "Didn't used to" would put the action in the past tense of the past tense, which makes no sense. If there is already a past tense verb in the phrase, then it's "use to", and it doesn't have to be negative: "He DID use to do that".
Personally, I've never grown completely comfortable with the "used to/did use to" construction and try to avoid it if possible. It's always seemed to me like a lazy short-cut to avoid precise English, however much it's used.
Almost nobody talks this way even if it's technically correct English. I mean, this translation would never have occurred to me under normal circumstances.
Maybe there was a different misspelled word in your answer? Note that you wrote "perfertly" instead of "perfectly" in your question here. I know that when I'm answering, my fingers run ahead of my brain a lot. In any case, yes, Duolingo should accept "he," "she," or even "it" (although the latter is highly unlikely).
I agree with benutzer. "Why did he not use to take the bus?" I would not say that. "Why didn't he/she take the bus?" would be the best option. (I had: Why not take the bus.)
Five long years ago someone suggested a better English translation, but obviously no one paid attention. No English speaker would say "Why didn't he use to take the bus?", so it should be changed to a more natural response. The current translation makes no sense. Thankfully, "Why didn't he take the bus?" is accepted as an alternative.
As the discussion here reveals, by requalifying translations as 'anything DL has heard is probably OK' we are opening the floodgates to FessbukSpeak. "Do you come here often? I didn't use to." Or is it "I didn't used to"? Or even "I didn't used too"? Or "I didn't yoused too" (Texas version). The possibilies are endless. Discuss for your GCSE English grade!
perhaps DL should tell us what he didn't use to take the bus. Was it a ticket? Was it a credit card? After more than 6 years of being told this sentence is rubbish, I think we ought to be told. Come on DL, prove to us that a human monitors your program at least once in 6 years!
No, my sentence is not seriously mangled. 'Hadn't used to' is correct English when referring to an action that used to happen regularly. However, people increasingly and incorrectly say 'didn't used to'. Your suggestion of 'Why didn't he take the bus' is correct English but it doesn't imply a regular action. It's more suited to describing a one-off action. How do I know? I'm a native English speaker who has earned a living as an editor for almost 30 years so please don't patronise me with 'Sorry, but'. Anyone who starts a sentence with that is anything but sorry.
My comment was in no way intended to be patronizing, my use of the word 'sorry' was a polite way of saying 'I do not agree with you if you chose to interpret it that way that is your problem entirely, The Imperfect in Italian can also be translated in this phrase as 'was taking / took not solely as used to take.
It is surprisingly hard to find on the internet actually and all the examples I've found for 'had used' or 'had not used' plus the infinitive are from classic rather than contemporary literature. However, the Oxford Handbook of English Grammar available via Google Books (page 256) 3rd paragraph down refers to it and quotes the positive example of: 'He had used to tell wildly funny stories'. It says its use is occasional and formal. I'm not sure if you'll be able to get straight to it through this long link. https://www.google.co.uk/books/edition/The_Oxford_Handbook_of_English_Grammar/azG-DwAAQBAJ?hl=en&gbpv=1&dq=%22had+used+to%22+grammar+examples&pg=PA256&printsec=frontcover Language does tend to evolve quite quickly and I suspect that it's used less and less often in everyday speech these days, which is why it may sound wrong to you. There's also a lot of conflicting information online.
I completely agree with your comment 4 months ago, when learning another language it is all too easy to fall into the trap of translating a phrase literally, we have all done it at some point. However you cannot apply English grammatical rules to another language, sometimes you just have to choose the best approximation and yet still ensure it makes sense.