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.

Good post. Something else has been on my mind for some time: your welcome/you're welcome/you're welcomed. Only once have I seen this addressed in a style guide, which suggested "you're welcome." So does this dictionary. But I've always used "you're welcomed." You are (were) welcomed to what elicited the thanks.
#1 Posted by Dan A., CJR on Tue 16 Aug 2011 at 02:20 AM
I see "a lot" a lot, but alot one word is not.
#2 Posted by David N, CJR on Tue 16 Aug 2011 at 12:09 PM
I disagree. "Alright" is a Briticism and should be avoided by Americans along with "Towards" and false cognates such as "pavement" (meaning sidewalk), "lift" (meaning elevator) and "tower flats" (meaning high-rise apartments).
#3 Posted by Stephen G. Esrati, CJR on Tue 16 Aug 2011 at 05:27 PM
David N. -
No, "alot" is NOT a word - only use "a lot".
#4 Posted by Lea-Ellen {night owl in IL}, CJR on Tue 16 Aug 2011 at 10:10 PM
Dan A. -
I believe that you wouldn't write or verbally say 'welcomed' in the context that you used "You're welcomed" in. 'Welcomed' is a past tense verb/adverb/past participle.
Correct Usages are :
You are welcome.
You're welcome.
She/he was welcomed into my home by my family.
You/she welcomed the guest for coming.
They welcomed me with open arms.
They were welcomed with open arms.
I was welcome to help myself to the food.
Let's welcome the arriving guests.
You are welcome to eat at our table.
We will welcome the new neighbors.
Always Incorrect:
You're welcomed.
Your welcome - Because of the mis-spelling of the word 'your'.
#5 Posted by Lea-Ellen {night owl in IL}, CJR on Tue 16 Aug 2011 at 10:17 PM
Hi, I always have a hard time remembering if a word has two of the same letter in it or not. Like the word "Happy" it has two p's.
Shimia
#6 Posted by vickie king, Sr., CJR on Thu 25 Aug 2011 at 04:47 AM
Why do the British say someone is "in hospital" instead of someone is "in the hospital"
#7 Posted by michelle , CJR on Sun 30 Sep 2012 at 01:07 AM