The differences between insure, ensure, and assure doesn’t have to do with their definitions. In fact, their definitions are quite similar – each word means “to make sure of something.” Their differences involve the way they’re used in a sentence.
The general rules for these three words are:
- insure - to take out an insurance policy on an item
- ensure - to guarantee that something happens
- assure - to convince another person that something is true
As you can see, each word has a specific use connected to its definition. And using these words interchangeably would be both grammatically incorrect and confusing to the reader.