Es/era improbable que + subjunctive

Question

For this question: Es improbable que los niños ________ la puerta. These options show as correct: hayan roto / rompieran. I understand hayan roto, as the impersonal beginning signifies "subjunctive", and the event is in the past. However, I learned at a language school in Cuernavaca that present tense statements (es improbable) could only match with haber + participle, and that only past events (eg "era improbable" could match with imperfect subjunctive (rompieran). The teachers made a big deal out of that. However here it shows up as a legitimate option. Can you explain? Thank you.

Accepted Answer

Hola Pat
It's possible that this structure is less flexible in Mexico, but in European Spanish both tenses are correct. The main clause can be placed at present time "es improbable que...", but the action expressed in the clause with the subjunctive can be referring to something in the past, and for that we can use both the Perfect tense or the Imperfect in the subjunctive. 
The nuance of the imperfect subjunctive in this case is that this action is seeing as less recent event while the perfect subjunctive implies a more recent event.
Saludos
Inma