But don't worry, it's not so difficult once you know some tips!
For example, "This is my each day lunch" would not work in the sentence, therefore, we know it must not be "every day" and should be "everyday."
You will also notice in the sentence above that the word "lunch" is a noun. That also immediately tells us we should use the adjective "everyday" to describe the noun.
Try these sentences below. Remember, try inserting "each day" first. Does it make sense? If yes, use "every day." Is there a noun after it? If yes, use "everyday."
(1) What should I do? I'm so hungry ______________.
(2) This is my ___________ life at work.
(3) Should I go swimming ____________ next week?
(4) ___________ is a new day.
(5) My _________ hairstyle is very boring.
Answers: (1) every day, (2) everyday, (3) every day, (4) Every day, (5) everyday