English insists on having variations of words, like “every day/everyday” or “any time/any time,” where two words are scrunched together in some uses, but must be separate in others. But people being people, the single-word version often shows up in place of the two-word version, and vice versa.
There are some basic guidelines: The one-word form is usually an adjective or adverb; the two-word form is usually a two-word phrase not modifying anything. But because that’s not always the case, it’s easier to just say the expression aloud. Are the desserts made “everyday” or “every day”? If you enunciate each word separately, it’s probably written as two words.
Just in case you can’t mutter aloud without attracting attention, here, for some of the most frequently abused cases, is a guide to when to use one word or two, with them used in context.
already (adv.), all ready (phrase): Hurry up and get all ready already! (If you’re talking about time, you want already; if you’re talking about preparedness, you want all ready.)
altogether (adj.) all together (phrase):We were all together, and we were altogether naked. (If you can say “all there” in the place of altogether/all together, you want all together; if you can say “completely,” you want altogether.)
anytime (adv.), any time (phrase): Come up and see me anytime you have any time. (If you can replace any time/anytime with “whenever,” you want to use anytime.)
awhile (adv.), a while (phrase): It took a while, but she could finally rest awhile. (If you can replace awhile/a while with a word like “silently” or “actively,” you want awhile.)
everyday (adj.), every day (adv.): Our everyday special is made in our kitchen every day. (If you can say “ordinary” in place of everyday/every day, you want everyday.)
everyone (pronoun), every one (phrase): In the bags of potato chips I bought for everyone, every one was broken. (If you can replace everyone/every one with “each one,” every one.)
onetime (adj.), one-time (adj.), one time (phrase): When he stole one time, he was a one-time thief; since he made a habit of it, but has since quit, he is a onetime thief. (While many dictionaries accept “onetime” for “one-time,” that hyphen can be useful when wanting to emphasize the singularity of the event.)
Eventually, the one-word versions will probably triumph. For example, the use of the adjective “everyday” in the place of the adverb “every day” shows up enough that it’s nudged up from a total no-no to Stage 2 on the five-stage Language-Change Index in Garner’s Modern American Usage.
Since it’s inevitable that they’ll be permanently fused at some point, as “onetime” has all but done, why should we bother to learn when to use one or the other?
Because we want to be “altogether” right, that’s why.