Content is one of those words that can be pronounced in two different ways, depending on the meaning.
Content (pron. CON-tent), meaning “the things that are held, included, or provided” (as in “The wine’s alcohol content is listed on the label”), is often used in the plural:
The table of contents is part of the book’s front matter.
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Content (pron. kən-TENT), meaning “satisfied,” can be used as a noun (“After I leave, you can sleep to your heart’s content!”), an adjective (“I’m content here.”), or a verb (“The show was sold out, so we had to content ourselves with a trip to the planetarium.”).
Contented is an adjective (meaning “satisfied,” as above, or “feeling or expressing satisfaction”):
“They don’t have chocolate ice cream. You will have to be contented with vanilla.”
Contentment is the state of being contented.
So when would you use content, and when would you use contented?
The words are synonymous, but according to Bryan A. Garner,* content is more common as a predicate adjective (“I am content just sitting here.”) and the adjective contented commonly precedes a noun (“The contented puppy fell asleep.”).
Adverb forms are contently and contentedly:
He lived contentedly among the natives.
The sheep were contently grazing in the pasture.
* Garner’s Modern American Usage, 3rd ed. (Oxford University Press, 2009).