• Assure means to tell someone that something will happen or that something is definitely true.

My doctor has assured me that I will be able to read without glasses after my surgery.

• To reassure somebody means to  make them feel less worried or frightened about a problem.

They reassured us that they would look into the matter.

• Ensure means to make sure that something will (or will not) happen.

Please ensure that all windows are closed and all doors are locked before you leave the building.

• Insure means to buy insurance to protect against something bad happening to you or your possessions.

You’d better insure your gear before you go hiking in the mountains.

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