Using "por si/por si acaso" for "in case" (subordinate conditional clause)

Por si and por si acaso are interchangeable. In Spanish we use "por si" or "por si acaso" to express "in case/just in case [something happens]".

They introduce a subordinate clause expressing a condition. They can be followed by the subjunctive or the indicative.

Here are some examples:

With the indicative - more probable

Me voy a llevar el paraguas por si llueve.I am taking the umbrella in case it rains.

Toma el dinero que te debo, por si acaso se me olvida después.Here is the money I owe you, in case I forget later.

Expliqué de nuevo la lección por si no les había quedado claro.I explained the lesson again in case they hadn't understood.

No debes decirle nada de esto a Roberto, por si acaso te pregunta.You must not tell Roberto about any of this, in case he asks you.

With the subjunctive - less probable

Me voy a llevar el paraguas por si lloviera.I am taking the umbrella in case it rains (less probability)

Toma el dinero que te debo, por si acaso se me olvidase después.Here is the money I owe you, in case I forget later. (less probability)

Expliqué de nuevo la lección por si no les hubiera quedado claro.I explained the lesson again in case they hadn't understood. (less probability)

No debes decirle nada de esto a Roberto, por si acaso te preguntara.You must not tell Roberto about any of this, in case he asks you. (less probability)

There are two things to consider:

1. We use the indicative or the subjunctive depending on what degree of probability we want to express. If what is said in the conditional clause after por si/por si acaso is considered by the speaker as "fairly probable" we use the indicative, but if it is considered more hypothetical and "less probable" then we use the subjunctive. 

2. The tense used in the main clause determines which tense to use in the clause after por si/por si acaso

Future action in main clause

If the main clause uses a tense that implies a future action, for example El Imperativo  or El Futuro Simple, then the por si/por si acaso clause uses El Presente (probable) or El Imperfecto Subjuntivo (less probable) 

Le daré el dinero que le debo, por si acaso se me olvida después.I will give him the money I owe him, in case I forget later.

Poneos las botas de agua por si lloviera.Put the wellies on in case it rains.

Past action in main clause

If the main clause uses a tense that implies a past action, for example El Pretérito Indefinido or El Pretérito Pluscuamperfecto, then the por si/por si acaso clause uses a past tense.

Le di el dinero que le debía por si acaso se me olvidaba después.I gave him the money I owed him just in case I forgot later.

Nos pusimos las botas de agua por si el camino estaba muy enfangado.We put our wellies [US: rain boots] on in case the path was very muddy.

Ella le había dejado la cena en el horno por si él no hubiera comido nada durante el viaje.She had left his dinner in the oven just in case he had not eaten anything during his journey.

Important:  El Presente de Subjuntivo is never used after por si/por si acaso. This would be incorrect:

  • Voy a llevarme el paraguas por si llueva.
  • No debes decirle nada de esto a Roberto, por si acaso te pregunte.

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Learn more about these related Spanish grammar topics

Examples and resources

Le di el dinero que le debía por si acaso se me olvidaba después.I gave him the money I owed him just in case I forgot later.
Nos pusimos las botas de agua por si el camino estaba muy enfangado.We put our wellies [US: rain boots] on in case the path was very muddy.
Me voy a llevar el paraguas por si lloviera.I am taking the umbrella in case it rains (less probability)
Ella le había dejado la cena en el horno por si él no hubiera comido nada durante el viaje.She had left his dinner in the oven just in case he had not eaten anything during his journey.
No debes decirle nada de esto a Roberto, por si acaso te preguntara.You must not tell Roberto about any of this, in case he asks you. (less probability)
No debes decirle nada de esto a Roberto, por si acaso te pregunta.You must not tell Roberto about any of this, in case he asks you.
Toma el dinero que te debo, por si acaso se me olvida después.Here is the money I owe you, in case I forget later.
Expliqué de nuevo la lección por si no les hubiera quedado claro.I explained the lesson again in case they hadn't understood. (less probability)
Le daré el dinero que le debo, por si acaso se me olvida después.I will give him the money I owe him, in case I forget later.
Me voy a llevar el paraguas por si llueve.I am taking the umbrella in case it rains.
Expliqué de nuevo la lección por si no les había quedado claro.I explained the lesson again in case they hadn't understood.
Poneos las botas de agua por si lloviera.Put the wellies on in case it rains.
Toma el dinero que te debo, por si acaso se me olvidase después.Here is the money I owe you, in case I forget later. (less probability)
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