Question about explanation and examples

Question

In the examples using past actions in the main clause, either the pretérito imperfecto or the pluscuamperfecto de subjuntivo are used. However, in the explanation it says the pretérito indefinido or the plusucmaperfecto.

Answer

Hola Stephen
The explanation is saying that the clause with por si acaso uses "past tenses" (different ones) - when we are mentioning specifically the pretérito indefinido and the pluscuamperfecto we are referring to the main clause. See this example in that block of information:

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.

Le di el dinero (main clause, using the indefinido),  por si acaso se me olvidaba (subordinate clause, using the imperfect [a past tense])
Saludos
Inma