So I think I understand that the reason we see the imperfect used for haber, when we form a past-perfect sentence like this, is that almost by definition, when we say anything like, "we had formed an opinion," we're talking about a state-of-existence -- the fact that we'd formed an opinion -- that would've been true continuously over a period of time. Regardless of when we're talking about, in the past, if, at that point, something had already happened, then it's having-happened-ness is a continuous fact. Hence, imperfect.
What I'm trying to figure out is whether there is any condition in which we'd want to use the preterite of haber (hube, hubister, etc) with a past-participle. Is that even a meaningful construction?
Hmm, Googling around, I seem to have answered my own question. It says here that the "preterite perfect" is very rare in modern use, but can be used for literary effect: http://spanish.about.com/od/verbtenses/a/compound-past-tenses.htm
I guess if you're specifying a point in time, you can use a past-perfect expression to define it in terms of what had just happened. Very interesting:
Cuando el niño se hubo dormido, el cura me pidió permiso para dejarnos. When the boy had fallen asleep, the priest asked me for permission to leave us.
Tan pronto hubo escuchado aquellas palabras, salió corriendo hacia la plaza. As soon as he had heard those words, he left running toward the plaza.
That doesn't really make any sense. The expression 'made up' is usually used in the sense that something has been fabricated, or not true, for example, 'When her mother asked her where she had been for the past three hours, she made up an excuse so she wouldn't get into trouble.' Or, 'I used to make up stories to tell my children at bedtime.' So, it wouldn't really work to say that 'When they asked us for our opinion, we made one up'. I hope that helps.
Dictionaries usually give several different meanings for every word. So Duolingo is giving us this extra information in case we are interested, without having to make the effort of actually looking it up! But we still have to figure out which meaning best fits the sentence given. (I have noticed that the owl usually prefers the first choice.)
If it can be of any help: to you:
Pretérito pluscuamperfecto y pretérito anterior comparados
El primero puede referirse a acciones pasadas que sucedieron varias veces y que no eran inmediatamente seguidas por otra acción. Además, suele aparecer en la oración principal. El pretérito anterior es todo lo contrario.
Cuando nosotros llegábamos, ya hacía tres horas que ellos habían terminado. (Esto ha pasado varias veces en el pasado, el pretérito pluscuamperfecto se encuentra en oración principal, y la distancia temporal entre LLEGAR y TERMINAR es máxima.)
Luego que hubo entrado, me miró. (Pret. ant. en oración subordinada, la acción sucedió sólo una vez, y la distancia temporal entre ENTRAR y MIRAR es mínima.)