Memory tricks will help you remember the difference.
Reader Christine wrote in with a request:
"I'd like to know the difference between the words insure and ensure, as in I-N-S-U-R-E and E-N-S-U-R-E. When writing, I often find myself backspacing so I can replace the word ensure with make certain and not feel like I've gotten it wrong. Thanks."
The Difference Between "Insure," "Ensure," and "Assure"
The verbs insure, with an I, and ensure, with an E, derive from the Latin word securus, meaning "safe" or "secure," which itself derives from se, meaning "without," and caru, meaning "care"—literally "without cares or worries." Also derived from securus are the English words sure, assure, secure, and security. On its path from Latin through French and into English, securus became simplified and took on a variety of prefixes, thus leading to the confusingly similar English verbs assure, ensure, and insure.
The verbs assure, ensure, and insure all have the general meaning "to make sure," and even though some argue that they are interchangeable (1,2), many maintain that their usage is dependent on context (3,4,5,6):
Assure is something you do to a person, a group of people, or an animal to remove doubt or anxiety, as in Squiggly assured Aardvark that he'd come to the party early. You can remember that assure can only be used with things that are alive (and both assure and alive start with a). Only things that are alive can feel doubt or anxiety, so only they can be assured.
Ensure is something you do to guarantee an event or condition, as in To ensure there'd be enough food, Aardvark ordered twice as much food as last year. You can remember that guarantee has those two e's on the end to help you remember that to ensure (with an e) is to guarantee something.
Insure can be done to a person, place, or thing, but it's reserved for limiting financial liability, most commonly by obtaining an insurance policy, as in Aardvark wondered if the caterers were insured against loss. You can remember that we take out insurance to protect our income if we become unemployed, disabled, or injured in an accident. Both insure and income begin with -in.
Finally, the related verb secure is used when you take possession of a thing or place, as in Aardvark secured a beautiful hall for his party, or when you make something stable or safe, as in Aardvark secured the welcome banner to the wall.