EVERYDAY and EVERY DAY

EVERYDAY is an adjective used to describe

something ordinary, commonplace, or used/occurring on ordinary days.

For example, "Her EVERYDAY clothes wear out quicker than her work uniforms."

For example, "His EVERYDAY routine includes coffee, the newspaper, and bocce."

EVERY DAY is a two-word phrase. In EVERY DAY, the adjective "every" modifies the noun "day," and it functions as an adverbial phrase. It can be used interchangeably with "each day."

For example, "I need to start going to the gym EVERY DAY."
For example, "Margie used to run EVERY DAY, but she stopped after her surgery."

The adjective EVERYDAY will modify nouns (like routine or clothes in the example above). Since EVERY DAY functions as an adverbial phrase, it will be modify adjectives, verbs, or other adverbs -- but not nouns.