I completely forgot about the appointment.
ap-pun-ta-MEN-to — stress the fourth syllable. Double 'p' is pronounced distinctly.
Use when you forgot a scheduled meeting or appointment. More formal than forgetting plans with friends — suitable for doctors, hairdressers, or casual professional meetings.
'Dell'appuntamento' uses the partitive contraction 'di + l'' contracted to 'dell''. 'Dimenticarsi di qualcosa' (to forget about something) requires the preposition 'di'. This is a fuller, more formal version of 'mi sono dimenticato'.
Ho dimenticato di segnarlo in agenda.
I forgot to note it in my diary.
Suggests it was an organisational failure rather than indifference
Non me lo ero segnato.
I hadn't noted it down.
Indirect — implies the fault is the lack of a note, not your memory
Mi era uscito completamente di testa.
It had completely slipped my mind.
Idiomatic and very natural — sounds genuinely forgetful, not careless
In Italian professional culture, forgetting an appointment is more serious than in some other countries. A genuine apology and an immediate request to reschedule is the expected protocol.