“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.)
At Writer’s Relief we proofread our clients’ work every day! If you want us to proofread your manuscript, we can!
Homework is due every day.
What is the terminology for comparing two words with one word of the same spelling?
Do you mean homographs? You may find this article of interest: https://writersrelief.com/2016/11/09/homographs-heteronyms-hononyms-decoded/