"Where were the fish yesterday?"
Translation:¿Dónde estuvieron los peces ayer?
07/23/18. Perhaps, but your fish would be swimming against the prevailing tide of Preterite tense examples, particularly because the Imperfect tense has not yet been introduced.
I now think they rejected estaban because "ayer" makes the time reference an enclosed one that is completed. But I think the same sentence could be correct with estaban if the surrounding context made it clear that the fish were there as a background condition while something else happened.
Estuvieron » preterite
Estaban » imperfect
Both are past tenses but have a difference in meaning.
Preterite » an action or event begun and completed at a definite past point in time. In this case, ayer.
Imperfect » an action or event that was habitual in the past, recurring over a period of time - eg, "The fish used to be here".
» Or something that happened at an indefinite time in the past, or the timing of when it did happen is not important, so there is no clearly defined start and finish time - "The fish were here" (at some time or other) - and may still be here.