Basically, insure, ensure and assure mean "to guarantee a person or something".

- Insure refers to regular certain payment to the insurance company so that your items are damaged, lost or when you are ill, died, they will give you or your heir the money. Insure is written as Ensure in American English. Assure is used in a formal context, refers that to make someone believe that something certain will happen or something is true.

- In addition to the meaning of paying insurance money like insure, ensure is used when someone is sure what will happen or be done. 

- Assure is used in a formal context, refers that to make someone believe that something certain will happen or something is true.  Assure is also used when someone does everything they can to do whatever happens.
E.g:
  - I look back to assure that no dogs are following me.
  - Our new system ensures that everyone gets paid on time.
  - The valuables are insured.

 

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