I find the trio of assure, ensure, and insure can turn into stumbling blocks since they sound so similar. However, they have slightly different meanings that must be taken into account.
Assure is typically used in the sense of to guarantee something: David assured his boss that the project would be completed by the deadline.
Related to this is the word reassure, which is to remove doubts or fears: His boss was reassured by David’s confident attitude.
Ensure is to check on something: David reviewed the blueprints to ensure the plans were being followed.
And finally, insure is related to provide insurance coverage: David’s final step of the project was to insure the building and its contents.