**First of all lets all agree that vergessen is a transitive verb which simply means that it has an object which in german is in the accusative form**


*Second of all lets also agree that objects in german can be ein Nomen, ein Pronomen, präpositionales Objekt,Infinitivsatz or Nebensatz**


es als Pronomen (obligatorisch)
=============
**A pronoun replaces a noun known for both parties involved, avoiding repetition. In other words it can not be object because it has grammatical role of filling the object place and a symantic role of making a connection or a link to the word it is replacing.**

example:

**Person #1: Ich habe das Alphabet vergessen.**

**Person #2: Waaaaaaas hast du es echt vergessen?**

here person #2 can also reply saying Waaaaaas hast du das Alphabet echt vergessen?
es > das Alpphabet

another example to better grasp the idea of the pronoun
Ich habe seine Handynummer vergessen?
Hast du sie eigentlich vergessen
sie > die Handynummer

es als (Verweiswort | Korrelat) (fakultativ)
=============
The general idea here is that we can remove the es without affecting the meaning at all and it only serves a grammatical purpose which is linking the second sentence to the first one 
lets state a couple of examples first and then discuss them

*example:*

**Person #1: Ich habe [es] vergessen, dich früher anzurufen.**

**Person #2: Hast du [es] echt vergessen, dass du mich anrufen solltest?**



**in both the first and the seccond examples the pronoun es serve in both sentences as a Korrelat which has the only purpose is pointing the upcoming subordinate sentence, and removing it will not affect neither the meaning nor the grammer because the subordinate is considered to be the accusative object itself**

es (unmöglich)
=============
*example* 

**Dich früher anzurufen, habe ich ehrlich vergessen.**

here the sentence is reformed so that the Nebensatz precedes the Hauptsatz, and in this case es can not both grammatically and symantically