These three English words all mean "to make sure or certain" and are commonly confused, even by native speakers. This lesson will ensure that you understand the difference.
Assure most commonly means "to remove doubt about, to guarantee, to set one’s mind at rest." It is a verbal statement of certainty. Assure is the only one of these three verbs that can be used with a person as the direct object.
Assure can also mean ensure (see below).
In British English, assurance is used as a synonym for a particular type of insurance (see below).
Ensure means "to make sure or certain" and must be followed by a direct object. Ensure indicates the action of guaranteeing, as opposed to the spoken guarantee indicated by assure.
Insure refers to the provision of insurance: coverage against a specified loss.
In British English, assure is used instead of insure when referring to insurance that is guaranteed to be paid (e.g., life insurance, because you are definitely going to die at some point), as opposed to car or fire insurance (which cover you against a loss that may or may not occur).
Insure can also mean ensure – to make sure of.
The Bottom Line
Assure, ensure, and insure can all mean "to make sure or certain of":
In my opinion, however, ensure is the only word that should be used here.
Beyond that meaning, assure also indicates a verbal guarantee, and insure refers to the action of guaranteeing coverage in the case of loss.