The difference between “everyday” and “every day” is quite simple, and there is an easy rule of thumb which you can use to determine which phrase would be suitable. In short, “everyday” is an adjective which describes something ordinary or commonplace, while the phrase “every day” means “each day.” Confusing these two is a very common grammatical error, and learning to catch this error can improve the quality of your writing immensely.

Given the brief description above, it should be fairly easy to determine which of these phrases is suitable. However, if you find yourself stuck, substitute “each day” and see if it works in the sentence. For example, if you can't decide whether the “train passes everyday” or the “train passes every day,” say “the train passes each day.” In this instance, the correct phrase is “every day,” because you mean to describe an event which occurs daily. If, on the other hand, you think the “passing of the train is an everyday occurrence,” you can see that “the passing of the train is an each day occurrence” simply doesn't work, so the correct word must indeed be “everyday.”

Instead of using “each day” as your litmus test, you can also substitute “ordinary” into a phrase where you think you want to use “everyday.” A sentence like “I go to the grocery store ordinary” doesn't sound right, so it must be “I go to the grocery store every day.” On the other hand, the sentence “a trip to the post office is an ordinary event” sounds perfectly normal, so the correct word is, in fact, “everyday.” By keeping the meanings of these terms in mind, you will find it extremely difficult to confuse them.

By being aware of the difference between every day and everyday, you may find yourself noticing these terms used erroneously a lot more, especially in grocery stores, which are notorious for curious grammatical errors. Many people, including the highly educated, don't always think out the words and phrases they use, so you can also encounter this common mixup in places like newspapers and even books, despite the fact that these publications are routinely edited by a team of people to catch such errors.

Incidentally, the word “everyday” appears to date back to the early 1600s, when it was used to describe ordinary, informal clothing, differentiating such clothing from formal clothing worn to church and major events. The sense of “ordinary” emerged around 150 years later, while the delightful everywhen and everyhow experienced a brief period of popularity in the 1800s, but alas never caught on.