The difference between 'Everyday' and 'Every day', and how to use them in sentences

The unpopularity of “every day” in this part of the world has given most English users, especially those on social media, the leeway to abuse its sibling, “everyday”. These words are not just different in meaning, but also in spelling.  “Everyday", which chiefly functions as an adjective, also functions as a noun. As an adjective, "everyday" is used to describe something that happens or is used every day, or something that forms a regular and basic part of your life, so it is not especially interesting or unusual, for example, everyday life, everyday clothes, everyday experience etc. 

In all of these examples, "everyday" functions as an adjective because it describes different nouns (life, clothes and experience). Therefore, it is very wrong to use "every day" when describing a noun. For instance, you don't write "every day experience" instead of "everyday experience".

Sentence examples
i. Don't let the problems of everyday life get you down.

ii. We are just an everyday family.

Although most dictionaries present "everydayness" as the noun form of "everyday", there are reputable online dictionaries where "everyday" adequately functions as a noun. As a noun, "everyday" means "the routine or ordinary day or occasion." 

American Heritage Dictionary

Dictionary.com

English Dictionary

Below is a sentence example from dictionary.com showcasing "everyday" as a noun.

"We use inexpensive plates for everyday."

In the example above, "everyday" means ordinary occasion and is functioning as a noun. Therefore, the sentence simply means that we use plates that are not expensive for ordinary occasion.

Unlike "everyday", "every day" solely functions as an adverb, that is, it qualifies a verb in a sentence. To be precise, it is an adverb of frequency because it tell how often an action is performed.
Examples
i. I wash my clothes every day.

ii. My everyday experience in the university taught me how to read my books every day.

In these examples, it will be wrong to replace "every day" with "everyday" because of the grammatical function of the latter. Whereas "every day" in sentence one tells us how often the speaker washes his clothes, the "every day" in sentence two describes how often the speaker reads his books. And such grammatical function cannot be performed by "everyday". Also, note that the use of "everyday" and "every day" in sentence two is not unintentional or for mere fanciful purpose. It is used to reiterate the fact that "everyday", which is an adjective, describes a noun whereas "every day", which is an adverb, qualifies a verb. Obviously, "everyday" in sentence two is qualifying the noun, experience, as against "every day" which qualifies the verb, read, in the same sentence.

Correct use of "everyday" and "every day"
The examples below show how you can use these words correctly.

i. Don't let your everyday worries make you fail the test.

ii. I don't get it. Do you mean you want to attend the party dressed in your everyday clothes?

iii. I eat rice every day.

iv. I have heard you. I will do what you said, but not every day.

Wrong use of "everyday" and "every day"
On the contrary, the examples below show how these words are used wrongly. Therefore, you should avoid them.

i. Don't let your *every day worries make you fail the test.

ii. This looks like an *every day attire.

iii. I eat rice *everyday.

iv. I will do what you said, but not *everyday.