Assure vs. Ensure vs. Insure

Assure means "to guarantee":

e.g. He assured her that he wouldn’t call his ex - girlfriend again.

Ensure means "to make sure by double checking":

e.g. The custodian ensured the doors of the school were locked at night.

Insure means "to provide insurance":

e.g. It is wise to insure your house against flood, fire, or theft.