These rules for verb tenses when using por si/por si acaso are really hard to keep straight. Does it sound really awful to a native speaker if we get it wrong?
Well, yes, it depends on how you get it wrong.
This constructions, that don't match with the English, are good to memorise as a whole, instead of analysing them too much: "por si + verb" // "por si acaso + verb" = in case [something happens]
If you find this construction a bit difficult to remember, there is an equivalent that is a bit similar to the English: in case...
en caso de que +verb
The difference is, this last one always takes the subjunctive, not the indicative.
The rule that "por si …" must never be followed by a present subjunctive, is logical - because the same is true in a subjunctive sentence which just starts with "si…" - i.e. it must follow the pattern: "Si + imperfect subjunctive > conditional". (I have to confess though, that I do sometimes forget that rule in the heat of conversation !) … But thank you, Inma, for telling us about the construction "En caso de que >" … Does this expression also require the use of the *imperfect* subjunctive? (and never the present one?)
In my dictionary I found the expression "En (el) caso de que venga …" as a translation of "In case he should come …" / "In the event of his coming …" - so yes, it seems that "En caso de que …" can indeed take the *present* subjunctive.
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