assure / ensure / insure
Telling someone, making certain, and covering a risk.
Assure means to tell someone something firmly so that they stop doubting it. Ensure means make certain of a result: ensure success. Insure means buy or provide insurance: insure the house. Do not use insure success unless a policy is meant.
Examples
Use I assure you the file is safe.
Use Booking the hall a year ahead ensures the reunion has a venue.
Use They insured the piano.
Avoid Booking the hall a year ahead insures the reunion has a venue.