After expressions like por si, por si acaso, and en caso de que, do we use the subjunctive or the indicative, or is it left to the speaker's discretion?

On a different note, can we use the subjunctive or the indicative after acaso (when it's used to mean "maybe")?

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    I think the subjunctive fits best when the point of view of the speaker is to explain some policy, opinion or action from the past, whereas the indicative fits better when describing a plan being proposed or carried out currently. Examples: Apagué la computadora por si hubiera una tormenta eléctrica. // Vamos a apagar la computadora por si hay relámpagos. Oigo truenos.* // You didn't ask about this, but I'll comment that I think "por si acaso" is used primarily in isolation. Aug 13 '19 at 6:39
  • I'd never use Por si hubiera, but maybe someone else would.
    – pablodf76
    Aug 13 '19 at 10:24

For por si and por si acaso, use the indicative:

Dejo la puerta sin llave por si llegas cuando no estoy.
"I'll leave the door unlocked in case you come when I'm not here."

Se quedó junto al teléfono por si acaso ella lo llamaba.
"He stayed beside the phone in case she might call him."

Note that in both examples we're dealing with hypotheticals, but the subjunctive is not used here. The word acaso just slightly emphasizes the contingency of the event. That's why I translated the second example as "she might call" instead of "she called". Other than that you could remove acaso and it would work the same.

I've found some uses of por si (acaso) with the subjunctive, but I'm not sure they're correct.

Note also that por si (acaso) works like a subordinating conjunction in these examples, but there's an extra use of por si acaso as an adverbial phrase which is used on its own, basically as it happens in English with "Just in case":

Por si acaso déjame la puerta sin llave.
"Just in case, leave the door unlocked for me."

Synonyms of this are por las dudas and the more colloquial por si las dudas, por si las moscas.

For en caso de que, use the subjunctive:

En caso de que no me encuentres, déjame un mensaje.
"In case you don't find me, leave me a message."

There's also para el caso de que, which sounds a bit more elaborate and correspondingly emphasizes the caution or foresight of the speaker. You can say en el caso de que (with el); it's not correct to say en (el) caso que (without de). In fast informal speech you might find the shortened form caso que.

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