Past action in main clause

Question

The instructions say to use "indefinido" but the indicitive mood examples use imperfecto. Also, the question asked on the quiz only has imperfecto and futuro simple as options. I went with imperfecto and it was marked correct.

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If the main clause uses a tense that implies a past action, for example El Pretérito Indefinido or El Pretérito Pluscuamperfecto, then the por si/por si acaso clause uses a past tense.

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.Nos pusimos las botas de agua por si el camino estaba muy enfangado.We put our wellies [US: rain boots] on in case the path was very muddy.

Accepted Answer

Hola Thomas
The lesson is saying when focusing on the past actions that if the main clause is using a past tense like the Indefinido then the clause with por si/por si acaso needs a past tense, so you have this sentence:
Le di el dinero que le debía por si acaso me olvidaba después.
Le di el dinero (using Indefinido in the main clause)
por si acaso me olvidaba (using the imperfect in the subordinate clause)
The same on this one:
Nos pusimos las botas de agua por si el camino estaba muy enfangado.
Nos pusimos (indefinido)
por si el camino estaba (imperfecto)
When we say that in these cases the por si acaso clause uses a "past tense", we don't mean the indefinido. It means a past tense as opposed to the clauses that indicates a future action where we use other tenses (present indicative or imperfect subjunctive).
I hope this clarified it.
Saludos