I left something in the taxi.
'Dimenticato' — 'di-men-ti-CA-to'. Five syllables; stress on the fourth.
Use when you exit the taxi and then realise you have left an item behind — say it immediately while the taxi is still there, or call the company.
'Ho dimenticato' is the passato prossimo of 'dimenticare' (to forget/leave behind). 'Qualcosa' means something. 'In taxi' means in the taxi. This phrase opens the conversation for describing what was lost.
Ho lasciato la borsa sul sedile.
I left my bag on the seat.
More specific — names the item and location.
Può tornare indietro?
Can you come back?
Urgent request if you have the driver's number.
Qual è il numero di targa?
What is the licence plate number?
Needed when reporting to the taxi company.
Italian taxis have a 'centrale operativa' (dispatch centre) that can contact the driver. Always keep your receipt — it contains the taxi licence number which is essential for recovering lost property.