Not if, but when

[Glen]

"It's not a matter of if [something might happen] but of when [it will happen]"

No es cuestión de si suceda o no, sino de cuándo va a suceder.

Sound alright?

[chileno]

Sí.

También " no es que si pasa, sino de cuando va a pasar"

Pasar= suceder = happen.

[Glen]

To emphasize the element of uncertainty (if, might) could it be No es que si pase o no,...?

[chileno]

Originally Posted by Glen

To emphasize the element of uncertainty (if, might) could it be No es que si pase o no,...?

Correcto, as a matter of fact I guess it should be only the subjunctive. Good catch.

[AngelicaDeAlquezar]

The formula can be used very colloquially (in Mexico), but without the subjunctive.
A Mexican might use it like this:
- No es que/de si pasa; va a pasar. Nomás hay que ver cuándo.
- No es que/de a ver si sucede; es de/que cuándo sucede.

...However, while writing these examples, this one came to my mind:
- No es que pueda pasar o no, sino cuándo va a pasar / cuándo pasa.
(I suppose that the uncertainty is actually expressed by "poder".)


In a less colloquial way, we would probably say something like:
- La cuestión no es si pasa o no, sino cuándo va a pasar.
- No se trata de si sucede o no; más bien de cuándo va a suceder.
- El asunto no es si puede pasar o no, sino que va a pasar alguna vez.
(There is no subjunctive, because conditional "si" doesn't need it in this case.)

[chileno]

Originally Posted by Glen

To emphasize the element of uncertainty (if, might) could it be No es que si pase o no,...?

Correct. After seeing Angélicas answer and re-reading yours, I saw that I mixed up my eyes/fingers with answering you.

No es que pase, si no, si va a pasar

Or like I said originally.

Check the "si" placement.

One is conditional and the other in the subjunctive.