assure, ensure, insure

Assure often means reassure someone. Ensure means make certain that something happens. Insure commonly means arrange insurance against a loss.

Reassure: I assured Maya that we had plenty of time.

Make certain: A tight lid helps ensure the soup stays in the jar.

Insurance: We insured the bicycle against theft.

These meanings overlap in established usage. In American English, insure can also mean make certain. Keeping ensure for that meaning and insure for insurance is a useful distinction when you want to avoid a moment of uncertainty.