Ensure means to make certain. Insure means to cover with insurance. Ensure the door is locked. Insure the car. Diction as a set is in word choice.
Make sure versus buy a policy
If the job is guaranteeing an outcome, use ensure. If money would pay for a loss, use insure. The ACT almost always wants ensure outside a finance sentence.
Elicit versus illicit is another look-alike, in elicit versus illicit. Affect versus effect is in affect versus effect. Grammar overall is in ACT grammar rules.
The bottom line
Make certain: ensure. Insurance: insure.
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