When, and why then, is "es" required with vergessen?

Question

In conversation with 2 native German speakers I said,

Gloria: »Ich erzählte Robbie Aschenputtel und ich habe vergessen, dass es Essenszeit war.«

Gloria's Mother: »Nun, das ist aber schade, dass Robbie auch vergessen hatte.«

and they corrected me to this:

Nun, das ist aber schade, dass Robbie es auch vergessen hatte.

In researching why "es" might be required here I entered the following into Google Translate:

I forgot. You forgot. He forgot. Robbie forgot.

and it returned the following:

Ich habe vergessen. Du hast vergessen. Er vergaß. Robbie hat es vergessen.

Doing the same thing in DeepL I got:

Ich habe es vergessen. Du hast es vergessen. Er hat es vergessen. Robbie hat es vergessen.

Since Duden has this sentence:

ich habe vergessen, was ich sagen wollte

clearly "es" is not always required; the verb can be used intransitively sometimes.

Now, why would "es" be required for "Robbie" and not "he"?

Answer 1

This is about whether “vergessen” needs an object. It does not always need one, but it needs one more than in English.

The sentence

Ich vergaß.

is grammatical, but it sounds very general. I forgot things. It does not say that I forgot something specific. So indeed the German speakers were right to correct you.

In your last example

Ich habe vergessen, was ich sagen wollte.

there is also an object, namely “was ich sagen wollte”, so this does not indicate that the “es” in your first example is optional.

To take up an example from another answer (slightly modified), no-one would answer

Hast Du Deine Hausaufgaben gemacht?

Mit “Ich habe vergessen”. Saying “Ich habe es vergessen.” instead would be less weird, but still a bit unusual, I think. Much more likely would be

Die/das habe ich vergessen.

Here “die” would refer to “die Hausaufgaben”, while “das” could be thought of as referring to the general “doing the homework”. Probably “die” would be a bit better. And in that case, the first word can indeed be omitted, and often will be omitted:

Habe ich vergessen.

Note however that the word order with the verb in first position shows that something in first position was omitted (and we only do that for an object like “das”), so in a sense “vergessen” still has its object here.

Answer 2

The verb "to forget" is transitive, but the object is optional when the meaning is clear without it. So in response to "Did you remember your homework?" you can say "I forgot," or "I forgot it." These expressions mean about the same thing. Similarly in German you can say either Ich vergaß, or Ich vergaß es. with about the same meaning. I think using a subject is more common in German, which is why you were corrected for not using it, but dropping it is also possible, which is why some of the machine translations did not include it.

Machine translators only give a possible translation, sometimes not always the best. So don't use them as evidence that a given phrasing is the only one possible. In other words, you can't tell from machine translations that es is required for "Robbie" and not "he". I prefer DeepL for translating into German because it gives many possible translations and you can usually adjust the wording to more closely match the meaning you have in mind.

On a side note, I'm pretty sure that in spoken German it would be more common to use the perfect tense: Ich habe es vergessen. But using the simple past isn't wrong.

Answer 3

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 semantic 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 Alphabet

another example to better grasp the idea of the pronoun

Ich habe seine Handynummer vergessen?
Hast du sie wirklich 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 example 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 semantically.