“Every day” is an adverbial phrase, where “every” describes “day.” Therefore: I eat an apple every day, and every day I feel better for it.
(Hint: If you can say “every single day” or “each and every day” then you should use two words: “every day.”)
“Everyday” is an adjective and must be used to describe an object: They were tired of their everyday clothes and longed to dress up. Or She filled her cart with everyday items, such as toothpaste, milk, and bread.
(Hint: “Everyday” must come before the object it describes.)
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