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.
There are parallels in English which can help remember what tenses to use after "por si acaso".
".... in case (in rhe event that) he arrives early... (present tense)" implies there's a fair chance he will arrive early.
".... in case (in the event that) he were to arrive early... (past subjunctive tense)" implies it's less likely he'll arrive early.
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