Everyday as one word is an adjective meaning ordinary or routine — everyday clothes. Every day as two words means each day — I walk every day. The test: if you can say "each day" instead, use two words.
The core difference
An adjective and an adverbial phrase, separated by a space.
- everyday (one word, adjective) — ordinary: everyday problems, an everyday occurrence.
- every day (two words) — each day: it rains every day.
The test
Substitute each day. "I check it each day" works, so it is two words. "Each day clothes" does not work, so everyday clothes is one word. The one-word form always sits in front of a noun, describing it.
The same pattern elsewhere
English does this repeatedly: anyone/any one, sometime/some time, everybody/every body, maybe/may be. In each pair the one-word form is a single unit of meaning and the two-word form is literal. Once you see the pattern, all of them follow.
Frequently asked questions
Which is correct: "I run everyday" or "every day"?
Every day, two words — it means each day.
What is the test?
Substitute "each day". If that works, use two words.
What other words follow this pattern?
Anyone/any one, sometime/some time, maybe/may be — the one-word form is a single unit of meaning.