Jtolaz, the problem lies in the fact that words we use in the English imperfect are often the same as the preterite. "todos los años iba a Cadiz" could be translated as either "Every year I used to go to Cadiz," or "Every year I WENT to Cadiz" with no change in meaning. I can also say "Ayer fui a Cadiz," translated as "yesterday I WENT to Cadiz."
Likewise, "tenía 8 años" is not translated as "I used to be 8." We say "I was 8," and the word "was" is the English preterite and also the imperfect of "to be", depending on context!
Bottom line, 'tenía' doesn't necessarily mean 'used to have'. The best way to view the imperfect in Spanish is as a macro or descriptive tense, in which preterite events can occur. When we translate the Spanish imperfect literally as 'used to', 100% of the time, we can end up with errors.
Not to be argumentative, because clearly you're making an effort to explain this (and believe me I do greatly appreciate any help I can get on this rather baffling topic area), but it still does nothing to clarify why in this case "had" is accepted but "used to have" is not. In English, I can't see any difference between them at all. I get that in Spanish, these tenses are used differently (how so I don't yet understand), but translating one into "had" and the other into "used to have" makes no sense to me.
It amazes me how many times a word that has two entirely different meanings in English has the same entirely different meanings in Spanish. Bajo means "under" as in being below something and is also used in the phrase "under control" which has nothing to do with being below something. Same for "segundo" which means "second" as is first, second, third and also means "second" as in a short period of time.
My book lists the following occasions to use the imperfect: Habitual or continuous actions in the past (used to); simple description..no ability to discern when something began or ended (Mary had a little lamb); "---ing" in the past...no mention of time.. (I was playing the guitar, and John was singing) (see below for progressive); mental or emotional action or physical sensation...a state of being in the past (you were tired); "would and could" in the past, but NOT when there is a condition...I would go to the movies, BUT I don't have time (conditional), I would go to the movies every weekend as a child (actually, I USED to go to the.....); clock time and ages in the past; había -- hay in the past: there was..., there were....; "Going to" do something in the past ir a in the imperfect; the progressive in the past Estar in the imperfect + gerund...I was speaking...Yo estaba hablando. The problem with Duo is there is no actual lesson.
The sentence I had before this was "Ellos tenian una bicicleta . . .," and now this is "Teniamos las cosas," and in both cases we're talking about having had a specific thing, so I don't understand why "used to have a bicycle" was OK but "used to have things" is not. I'll report it, because I don't see a contextual difference here.
I think they should add a time frame / context to the sentence. If for instance it was "When we were young" then 'used to' would clearly make sense. On the contrary, I'm wondering what would be an example for a context where you would use imperfecto in spanish, but 'used to' doesn't work to translate it?
The way that I was taught to translate by my Spanish teacher (for A-level) was to "Get an understanding of what the sentence means, throw that meaning up in the air, and then get it to make sense in English"; I would be marked down for doing literal translation of sentences that make little sense in English, for example, the Spanish phrase "Los hombres infieles tienen un cociente intelectual más bajo" literally would be "The men faithless have a coefficient intellectual more short" (This is from a random study that I took off of the internet) which is nonsense in English, and I would be penalised for translating it poorly, the correct way to translate it would be closer to "Faithless men have a lower intelligence quotient.".
You get a lingo because I can't a get a cup of coffee with them and because it annoys me when people ask why we/they/I cannot make Spanish comport with what English does or what I want or what I like or what I mean. And of course your answer was correct. Sorry,,,prickly day