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Question

Hola, lo siento..This example from the lesson looks like a precaution (just in case) yet uses the  imperfect indicative.  Muchas gracias, Shirley. 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.

Silvia Píriz · Accepted Answer

Hola Shirley S.
That's a great question. Although por si acaso often expresses a precaution ("just in case"), it does not always require the subjunctive.
In your example:
Le di el dinero que le debía por si acaso se me olvidaba después.
the speaker is referring to something they considered a real possibility at that moment: I might forget later. Since the possible event is presented as a realistic possibility rather than a hypothetical or uncertain future event, the imperfect indicative (olvidaba) is natural here.
Compare it with:
Te dejo una copia por si acaso la necesitas. (indicative: a real possibility)
Llévate un paraguas por si acaso llueve. (indicative: it may rain)
The subjunctive is generally used when the clause refers to a future event that is presented as more hypothetical or dependent on a condition, but por si (acaso) most commonly takes the indicative because it introduces a possible reason for taking a precaution, not a condition.
So in this sentence, se me olvidaba is correct because it reflects a genuine possibility the speaker wanted to guard against.
Saludos
Silvia

Silvia Píriz · Answer

Hola Shirley S.
That's a very good observation. The key difference isn't really "more or less probability", but how the speaker presents the situation.
In your new example:
Expliqué de nuevo la lección, por si no les hubiera quedado claro.
the speaker is referring to something that may already have happened (perhaps they didn't understand) but whose result is unknown. Because the clause expresses a hypothetical past possibility, Spanish uses the pluperfect subjunctive (hubiera quedado).
Compare it with:
Expliqué de nuevo la lección por si no les había quedado claro.
This is also possible, but it presents the lack of understanding as something the speaker considers more factual or likely, rather than simply a possibility.
So, rather than thinking in terms of more or less probability, it may be more helpful to think of it this way:

Indicative → the speaker treats the situation as a real or plausible fact.
Subjunctive → the speaker presents it as hypothetical, uncertain or simply a possibility without committing to it.

In practice, with por si, you'll find both moods in different contexts, because the choice often reflects the speaker's perspective rather than a strict grammatical rule.
I hope that makes the distinction a little clearer!
Saludos, Silvia

Shirley Shostak · Answer

Thank you for this Inma. I am still struggling  to know when there is more or less  probability. Eg,  in this example from the lesson I would not be able to fill in the blank if hubiera was left blank.: 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)Muchas gracias, Shirley.