
entfallen
to be canceled
Not to take place; to be canceled or called off.
Wegen Krankheit der Lehrerin ist der Unterricht heute entfallen.
Because the teacher is ill, class was canceled today.
to be canceled / to slip one's mind / to fall to
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to be canceled
Not to take place; to be canceled or called off.
Wegen Krankheit der Lehrerin ist der Unterricht heute entfallen.
Because the teacher is ill, class was canceled today.

to slip one's mind
To be forgotten by someone; to disappear from memory.
Mir ist leider Ihr Name entfallen.
Unfortunately, your name has slipped my mind.

to fall to
To fall to someone or something as a share; to be allocated.
Der größte Teil der Kosten entfällt auf das Material.
The largest part of the costs falls to the materials.
'Entfallen' (to be canceled/to slip one's mind/to fall to) is an inseparable verb. The prefix 'ent-' remains attached to the verb stem in all sentence forms and tenses and is not placed at the end of the sentence.
Ich hoffe, dass die Prüfung morgen nicht entfällt.
I hope the exam isn't canceled tomorrow.
The perfect tense of 'entfallen' (to be canceled/to slip one's mind/to fall to) is always formed with the auxiliary verb 'sein' (to be). This is because the verb describes a change or a disappearance.
Die letzte Stunde ist gestern entfallen.
The last class was canceled yesterday.
In the meaning 'vergessen werden' (to be forgotten), 'entfallen' (to slip one's mind) is used with the dative. The person who forgets something is in the dative, while the forgotten information is the subject of the sentence.
Mir ist die Adresse entfallen, kannst du sie mir nochmal sagen?
The address has slipped my mind; can you tell it to me again?
When 'entfallen' means that a share falls to something, it is always used with the preposition 'auf' and the accusative. The structure is: share + entfallen + auf + whole.
Von den Einnahmen entfällt ein Drittel auf die Steuern.
Of the revenue, one third falls to taxes.
to forget
Both words can express that information is no longer in someone’s memory. With vergessen, the person is the subject and the information is the object. With entfallen, the information is the subject and the person is in the dative; this form sounds more elevated.

Der Name des neuen Kollegen ist mir entfallen.
The new colleague’s name slipped my mind.
Ich habe den Namen des neuen Kollegen vergessen.
I forgot the new colleague’s name.