"I didn't realise it was dangerous there"
Is this:
no me di cuenta de que fuera peligroso ahí
or
no me di cuenta de que era peligroso ahí
?
Or something else?
Should you always use subjunctive with darse cuenta in the past?
2 Answers
I think Daniela´s explanation is reasonable although not just nuance but time factor is also important to determine the choice between subjunctive and indicative in this case.
I would say that "no me di cuenta de que fuera peligroso ahí" indicates that in the past I didn´t realize the danger that would happen (realization first - danger second. Basically the same as "No me di cuenta de que iba a ser peligroso").
"No me di cuenta de que era peligroso ahí" indicates that in the past the discovery of the danger and the realization happened at the same time.
The difference in nuance between the two is so minute that there are probably many people who really do not take the time to to choose - we just say what comes to mind and everything is ultimately understood and only a grammarian such as a former SD member Lazarus would even notice.
I would go with your first answer, because it introduces a negative.
A positive statement with darse cuenta would not require the subjunctive.
With regard to the negative I have seen it stated both ways by native speakers, (which also I am) and I have even seen arguments over this problem. Many people would use the indicative.
My feeling is that is a matter of emphasis. If I am trying to express that "it might have been dangerous" Then I would use the subjunctive.
If I am emphasizing the fact that it was dangerous, then I would use the indicative.
This is one of those areas where the rules are not so rigid.
You will probably see a few different opinions pop up here. 
