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I learned that "just in case" should be translated por si acaso, but I have also heard por si used by itself without the acaso. I believe I've even heard si acaso without the por. What is the difference? Is it just stylistic, or are the cases where you must use one or the other?

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up vote 3 down vote accepted

In principle, the three combinations have different meanings:

  • Por si acaso is an expression which adequately translates just in case.
  • Por si might be a shortening of por si acaso. Perhaps you heard por si las moscas, and couldn't make sense of it. That is a slang variation of por si acaso. A couple of examples where it is used as a shortening of por si acaso are:

    Voy a hacer (algo) por si (algo).

    Cerraré la puerta con llave por si hay ladrones merodeando.

    Ahorré dinero por si llegaba la crisis económica.

However, I believe this is not a correct use of the language, though it might be common in some places. It doesn't sound right to me, at least.

  • Similarly, si acaso sounds like a slang variation and not entirely correct (actually, it sounds worse than por si).

    Cerraré la puerta si acaso hay ladrones merodeando.

    Si acaso vienes a visitarme, cocinaré pavo para ti.

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I noticed "Si Acaso Vuelves" is the name of a Vicente Fernandez song (and I'm pretty sure I've heard it in a different song as well). – jrdioko Feb 3 '12 at 3:49
Si acaso is incorrect in this context. But it is correct in other contexts. – Joze Feb 3 '12 at 7:36

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