As a teacher, I saw students often confuse the terms everyday and every day, mostly because they were unaware of the grammar involved. And as an editor, I see this mistake in native speakers’ writing all too frequently. It’s not only a matter of spelling; the spelling reflects the grammar rules that must be adhered to. But fear not! The grammar rules are easy to remember, and if you read through this post, you and your students will most likely never make this mistake again.
Rule
If you’re unsure which form to use, look for the noun. Is there a noun that follows? If so, use everyday. If not, use every day.
Everyday, spelled as one word, is an adjective that must come before a noun (see the note below concerning this adjective following the be verb).
Every day, spelled as two words, is a time expression that is best defined as an adverbial expression. It takes the same sentence position as many adverbs and adverb clauses do, namely at the beginning or end of the sentence or clause. It is technically an adjective and a noun (but note that some grammar books, unlike most dictionaries, classify every as an article or quantifier), though together these terms take on an adverb function.
In English, most adjectives can either come before a noun or follow the verb to be (e.g., She is intelligent. The painting was beautiful.) However, the adjective everyday will not occur in this position (e.g., The practice session is everyday at 6:00 pm. = incorrect). When we use the be verb in this case, the meaning is about the time or frequency, so we should write the time expression every day (e.g., The practice session is every day at 6:00 pm. = correct). It’s a good idea to point this out to students so that they won’t make the understandable mistake of using the adjective form after the be verb for this case.
You can see examples of these terms in use right in the titles of these ESL Library's sections: Everyday Idioms and Everyday Dialogues.
I really should (and will!) write a blog post dedicated to the different time expressions in English: adverbs of frequency (e.g., always, sometimes), adverb clauses (e.g., when + SVO), and adjective + noun (e.g., every day, once a week). Coming soon!
Wishing you happiness every day,
Tanya
Anonymous A.(Teacher)
July 15, 2015 at 7:49 am

Tanya Trusler(Author)
July 15, 2015 at 9:23 pm
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