Still confused

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

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.