"lo" vs "la"

[lukewink]

I was talking with a native Spanish speaker who used the phrase: "no la vas a creer" (you're not going to believe it)

I asked her why use "la" instead of "lo" in that sentence, and she wasn't sure why. In fact, she said you would use "lo" in the sentence: "no lo voy a creer" (I'm not going to believe it).

Can anyone explain why one sentence uses "la", and the other uses "lo"?

[AngelicaDeAlquezar]

Don't worry much about that... "la", is colloquially used instead of a more grammatically correct "lo", as an immediate impulse, sometimes being related just to what the speaker had in mind at the moment.

No la vas a creer. -> The speaker may have in mind something like "la historia", "la mentira"... even if he/she is not aware.
No lo vas a creer. -> It expresses a more general idea about the situation.

[lukewink]

Thank you!

[Ronald Ross]

One reason for using "la" with "creer" would be if the direct object is feminine.

[1] Esa historia no la creo.

Then too usage varies between Peninsular Spanish and Latin American Spanish. As explained in the Nueva gramática de la lengua española published by the Asociación de Academias de la Lengua Española (2009), in Spain the tendency is to make the direct object of "creer" the person. It is the person that is believed or not. Here in Latin America, it is what was said that is not believed, so the person who said it is represented by an indirect object:

[2] No se lo creo.