lessen / lesson are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
lessen: NA
lesson: (noun) a unit of instruction. (noun) a task assigned for individual study.
lessen: l·EH·s·ah·n
lesson: l·EH·s·ah·n
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)