Why is this sentence subjunctive?
Cuando todos los ingredientes estén mezclados, refrigéralos por dos horas.
Once all of the ingredients are mixed, refrigerate them for two hours.
It was a model sentence in SpanishDict.com's word-of-the-day today, so I'm confident that it is correct. We can put it into a more conventional form, with the dependent clause second, if that will help us analyze it:
*Refrigéralos por dos horas cuando todos los ingredientes estén mezclados.
There is no uncertainty, no emotion, no suggestion. It doesn't seem to fulfill any of the conditions for subjunctive. The only other use for estén (I believe) is as a command (imperative), but the cuando clause isn't a command.
I'm no grammar expert, but I would think because "once" is a conditional concept that that is the reason for the subjunctive. It actually hasn't happened yet, it is conjecture, that is the uncertainty. :) Even though you will be mixing those ingredients you actually haven't yet. Once (and if) you mix them, then you refrigerate.
Also, this guide looks pretty good: http://notesinspanish.com/files/NIS-Super-Simple-Subjunctive.pdf
That's pretty funny, huh? :) El tigrito might like that in his memory palace. ;)
Some adverbial clauses in Spanish either always require subjunctive or require it when speaking about things that haven't happened yet. Some textbooks refer to these as A SPACE--always require (past and present)--and THE CD--require for the present.
There's information here: http://users.ipfw.edu/jehle/courses/SUBJADV.HTM.
And someone made a quiz here: http://www.spanishdict.com/flashcards/27180/expresiones-utiles-aspace-the-cd