These six verbs overlap, but they focus on different things: what someone believes, what happens, or what receives protection.
- Assure means to tell someone confidently that something is true or will happen. It usually takes a person as its object: I assured Maya that the files were safe. You normally assure someone that something is true or of something.
- Ensure means to make certain that something happens or is true. The focus is the result, not the listener: Please ensure that every window is closed.
- Insure usually means to arrange insurance against financial loss: We insured the house against fire. It can also mean ensure, especially in American English, but choosing ensure for general certainty keeps the distinction clear.
- Reassure means to help someone feel less worried: The doctor reassured me that recovery would take time. Unlike assure, it specifically focuses on reducing worry; it does not simply mean saying something again.
- Guarantee can mean to make a firm promise: The shop guarantees repairs for a year. It can also mean to make a result certain: Practice does not guarantee success.
- Secure can mean to obtain something, often through effort: They secured funding for the project. It can also mean to protect something: Use a strong password to secure your account.
Compare She assured me that delivery was arranged with She ensured that delivery was arranged. The first reports confident words; the second says she made the arrangement certain.