"Es importante que se use el subjuntivo… pero cuándo?"
Es importante que se use el subjuntivo pero cuándo?
Is this correct? I want to say
'It is important to use the subjunctive... but when (do you use it)?
Sí. Es correcto.
Since you used an impersonal expression, "es necesario," the use of the subjunctive is correct.
If you are saying "it is important TO USE the subjunctive", the correct way to say it is: "Es importante usar el subjuntivo
If you are saying "it is important that one use the subjunctive, then what you said was correct.
Most impersonal expressions that act as subjunctive "triggers" such as 'es importante', can be used as above when it is being used as a generality and there is no subject that it refers to."
"Cada vez que sea necesario"
You can find the details of when it's necessary in the Learn Spanish section of the site.