But the example you gave, "volverlo a ver" is interesting because it's the infinitive of an expression requiring an infinitive. And an infinitive or a gerund cannot have an unstressed pronoun before, it can only go after the infinitive. When there are several enchained infinitives like that, the infinitive of an expression followed by an infinitive has itself those two possibilities, but since the pronoun cannot go before an infinitive, you either put it after the main first infinitive of the "modal" verb or after the second infinitive. That possibility is added to the one where the stressed pronoun goes in the beginning, before the main verb all the infinitives depend from. So an expression such as "I want to do it again" can be said in these three fashions using the "volver a+inf" expression: Lo quiero volver a hacer=Quiero volverlo a hacer=Quiero volver a hacerlo First you have the option "Lo quiero volver a hacer". Then the second option is putting it after the infinitive as usual, but since the infinitive is already an expression with an infinitive itself, that options has itself two possibilities too. And since the pronoun cannot be before an infinitive, those possibilities are "volverlo a hacer" and "volver a hacerlo". "Quiero lo volver a hacer"* is wrong. Only those three possibilities are allowed.

Todo depende de la oración. Por ejemplo, si hablas de una película: "Voy a volver a verla" o "Voy a volver a ver la película" Creo que depende de como utilice el objeto de la oración. Si usas "verlo" o "verla" se supone que estás hablando del objeto, si usas "ver" tienes que explicitar ese objeto. Saludos

Both options are equally correct and equally used in the language. The group of unstressed pronouns can go in both positions. That's so with most verbal expressions that require an infinitive, both when the infinitive is the direct object and when it's preceded by a preposition. The rule also works with estar+gerund, ir+gerund and venir+gerund. When the first part is not an infinitive, but a conjugated verb, there's these two possibilities: Voy a verlo=Lo voy a ver. Estoy buscándolo=Lo estoy buscando. Puedo decírtelo=Te lo puedo decir Quiero arreglarlo=Lo quiero arreglar Trato de prepararlo=Lo trato de preparar Volveré a hacerlo=Lo volveré a hacer Voy perfeccionándolo=Lo voy perfeccionando Tiende a deteriorarse=se tiende a deteriorar Both possibilities are equally correct, but there's no other possibility. "Quiero lo hacer"* is wrong, you say either "Lo quiero hacer" or "quiero hacerlo". To be continued...