Me explicó su porqué pero no le creí. She explained her reason but I didn't believe her.
Muchas gracias,Shirley.

Question

Indirect object pronoun:Generally you ask the verb "what/who?" and that gives you the direct object and you ask the verb "for whom, to whom?

I’m confused, is there another way to determine when to use a direct or indirect pronoun? Eg, how do you apply “for whom, to whom” in this sentence:

Accepted Answer

Hola Shirley (and David) 
I can't see that sentence in this Kwiziq lesson or any of its questions, but using "le" is correct.
As David explained, you can interpret it in two different ways, you can be referring to "something" that you believed (lo,la = direct obj. pronouns) or you can be referring to believing "the person" (le = indirect obj. pronoun).
If you have a read here at what RAE says about the verb "creer" and the use of pronouns with it, in number 3. it says that if "the something" is not explicit, you can use both the direct or the indirrect pronoun to express that you do/do not believe the person (le or lo/la). In this case, although you can take that "su porqué" as the "something" he didn't believe (he didn't beliieve it - as David said), we don't really see in the sentence what the reason was, it's not explicit, therefore the "le" in "no le creí" is referring to not believing "him/her" (indirect obj pron), but if we consider as I said that the thing is not explicit, you may also use the direct obj. pronoun "la/lo": "Me explicó su porqué pero no lo/la creí.",still meaning not believing him/her.
Saludos y Feliz Año.
Inma

Answer

Hi there Shirley -The rules which you quote are good ones.However, my Oxford Dictionary does indeed suggest "Tú le crees?" as the translation of "Do you believe her?", and it is true that this seems to be a [rare] departure from your rule.So perhaps it is best to regard "creer" as an unusual case, an exception - because [in English as well as in Spanish] there are two completely different contexts in which this verb can operate:[1] Believing or disbelieving an apparent fact ...and [2] Believing or disbelieving a person, which is really a  case of disagreeing with them - and it just seems that [most?] Spaniards would prefer to regard this as an indirect object.In the example which you quote, we could focus instead on the "porqué" >i.e., replacing the original sentence by "... but I did not believe it", which would then translate as "... pero no lo creí".An interesting illustration in my dictionary is "No le creo ni una palabra" = "I don't believe a word she says" - containing both a direct and an indirect object.