The verb lessen means to decrease or reduce.

The noun lesson means an instructive example, a piece of practical wisdom, or a unit of instruction.

Examples:

• "Lying increases the creative faculties, expands the ego, and lessens the frictions of social contacts." (Clare Boothe Luce)
• "The chief lesson I have learned in a long life is that the only way to make a man trustworthy is to trust him; and the surest way to make him untrustworthy is to distrust him and show your distrust." (Henry L. Stimson)

Observation:

"And how many hours a day did you do lessons?" said Alice, in a hurry to change the subject.

"Ten hours the first day," said the Mock Turtle: "nine the next, and so on."

"What a curious plan!" exclaimed Alice.

"That's the reason they're called lessons," the Gryphon remarked: "because they lessen from day to day."
(Lewis Carroll, Alice's Adventures in Wonderland, 1865)

Practice:

(a) During the recent financial crisis, central banks tried to _____ the effects of the economic downturn by lowering interest rates.

(b) "Life is a long _____ in humility." (James M. Barrie)
Answers to Practice Exercises

Glossary of Usage: Index of Commonly Confused Words