"Ensure," "insure," and "assure"

Ensure, insure, and assure all mean to make a thing or person sure. They are interchangeable in many contexts where they indicate the making certain or inevitable of an outcome.

Native speakers of English also have trouble with these words. Here is some advice given in Merriam-Webster's Collegiate Dictionary:

Ensure may imply a virtual guarantee: ensured the safety of the refugees

Insure sometimes stresses the taking of necessary measures beforehand: insure the success of the party

Assure distinctively implies the removal of doubt and suspense from a person's mind: assure you that no harm will be done