The Difference Between "Insure," "Ensure," and "Assure"
Guest writer Charles Carson answers,
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. (来源：英语交友 http://friends.englishcn.com)
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.
Quick and Dirty "Ensure," "Assure," Insure" Tip
So the quick and dirty tip is to use assure for things that are alive (remember that a is for alive), ensure to guarantee events and conditions (remember those two e's at the end of guarantee), secure for things and places, and insure for all of the above in financial contexts (remember the i is for income).
What About "Reassure"?
But wait, there's more. English has added its own prefixes to generate new words. The verb reassure is largely used interchangeably with assure, but it should be reserved for situations in which the assurance is repeated or ongoing. Reassure can also be used when a previously held belief is later doubted—that is, a person who was once sure of something can be reassured if they later experience doubt. For example, one might say Tom's faith in the justice system was tested by the proceedings, but the judge's final decision reassured him.
How About "Assurance," "Reassurance," and "Insurance"?
Then there are the noun forms. Assurance, reassurance, and insurance follow the same rules as their verb counterparts; use assurance and reassurance when eliminating doubt or anxiety in things that are alive, and use insurance (with an i) in financial contexts.* Unfortunately, there is no word ensurance (with an e), so writers must work around it with phrases like attempts to ensure, as in Despite Aardvark's attempts to ensure there'd be food for everyone, he forgot that Squiggly doesn't eat bugs.†
Thanks to Charles Carson, managing editor of the journal American Speech, for guest-writing this episode; and thanks, Christine, for your question.
*Although usually used only for formal policies underwriting financial risk, insurance is sometimes used euphemistically in criminal situations involving money, such as cases of bribery or extortion, as in