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

Learn about por si/ por si acaso in Spanish

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 (careful!! Not the present subjunctive) or the indicative.

Here are some examples:

With the indicative - more probable

With the subjunctive - less probable

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 or a mood 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 Pretérito Imperfecto Subjuntivo (less probable) 

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.