assure / ensure / insure
Telling someone, making certain, and covering a risk.
Assure takes a person: you assure someone that something is true. 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
UseI assure you the file is safe.
UseChecks ensure the count is right.
UseThey insured the piano.
AvoidThey insured the count was right.