"Algún día lo voy a olvidar."
Translation:Someday I am going to forget it.
"lo voy a olivdar" and "voy a olivdarlo" are both fine. However "voy a lo olvidar" would seperate the verb phrase, so isn't used
I've found this myself. Whenever an object pronoun is referred to, in this instance 'it', in a sentence with an infinitive verb, in this instance olivdar, it goes before the conjugated verb, not the infinitive verb, in this instance, voy.
Hope someone else can clarify that?