Subjuntivo pasado

Question

Hola…..¿por qué se usa el imperfecto de subjuntivo con una oración que empiece con “No sabía que_________”.  No entiendo el “trigger” en este caso.

Accepted Answer

Hola Barbara

The reason why the imperfect subjunctive is used in a sentence that starts with "No sabía que" ('I didn't know that') is because the structure implies a lack of knowledge in the past regarding the certainty or realization of the action in the subordinate clause. By using the imperfect subjunctive, it expresses a hypothetical, uncertain, or unconfirmed condition at the time referred to in the main clause.
For example:
No sabía que ella estudiara medicina.
Translated, this would be: 'I didn't know that she studied medicine'.
Here, the use of the imperfect subjunctive ("estudiara") indicates that in the past, there was no knowledge about whether she was really studying medicine or not. It is a construction used to express doubts, lack of knowledge, or unconfirmed situations in the past.
I hope this explanation clarifies your doubt.
If you have any more questions, feel free to ask!
Saludos
Silvia

Answer

Hola Barbara
Adding to Silvia's answer, I would also mention that there is a specific nuance in sentences using:
No sabía que... 
When we use this with the imperfect subjunctive, the message behind that "I didn't know that..." also contains the nuance of "I'm surprised to find out that..." as if it's more difficult to "accept" that new knowledge (like when we say "Me sorprende/Me sorprendió que... subjunctive]) 
You could also use the indicative with "no sabía que..." if what comes after is something that you "completely accept": 
No sabía que ella estudiaba medicina.
No sabía que estabas tan enfermo.
No sabía que ibas a venir.
With all these using the indicative there is more acceptance of these actions, we see them as more real, a realisation that they are/were in fact happening (...although I didn't know they were happening in the first place)
Un saludo
Inma