Many common words and phrases are identical except for a strategic letter space — apart and “a part” come to mind — and though knowing which form to use in a sentence is often obvious (as in that example), the difference can be subtle. Here are some of the less clear-cut pairs:
1. Ahold/a hold: Ahold is a variant of the noun hold, used in such sentences as “I’ll get ahold of you later.” But when you mean to refer to an actual grip on something, use two words: “She really has a hold on you.” If it’s hard to decide which form to employ, try this test: If you can insert an adjective between a and hold, the two-word form is appropriate.
2. Already/all ready: Use the former when you need an adverb, as in “I told you already.” The latter form is correct in sentences such as “We’re all ready for the party.”
3. Alot/a lot: These two forms are interchangeable except in one significant respect: The one-word version is wrong. It is used often in informal writing and may one day be standard, but until you get the official memo, refrain from using it if you want to be taken seriously as a writer.
4. Alright/all right: See item number 3.
5. Altogether/all together: The one-word form, an adverb, suffices to mean “completely” or “in total,” as in “Altogether, we saved $100 on the deal.” (It also means “nude” in the idiomatic phrase “in the altogether.”) The phrase is appropriate for sentences such as “We are all together in this.”
6. Anybody/any body: The one-word form is a pronoun used in such constructions as “He doesn’t get along with anybody.” The two-word adjective-noun form is applicable in limited contexts, such as in the sentence “Any body in motion responds to gravity.”
7. Anymore/any more: The one-word form is used as an adverb in sentences such as “We don’t go there anymore”; the two-word form consists of the adjective any and the noun more, as in “I just can’t eat any more of that pie.”
8. Anyone/any one: The one-word form is a pronoun, synonymous with anybody, used as in “Anyone can make that claim.” “Any one” consists of the adjective any and the noun one, as in “Any one of you might be next.”
9. Anyplace/any place: The adverb anyplace is a synonym for anywhere: “She won’t let me go anyplace without her.” The latter usage is an adjective-and-noun phrase that describes a location: “He doesn’t want to go to any place he can’t smoke.”
10. Anything/any thing: Anything is the likely usage: “I don’t remember anything.” The two-word adjective-noun form is generally separated by an another adjective: “She’s just does any little thing she wants.”
11. Anytime/any time: To describe with what frequency something might occur, use the one-word adverbial form: “Stop by anytime.” The two-word adjective-noun form is preceded by the word at: “You may leave at any time.”
12. Anyway/any way: Anyway is a synonym for anyhow: “We didn’t want to go anyway.” The two-word adjective-noun form is preceded by the word in: “That doesn’t change the results in any way.”
13. Awhile/a while: The noun phrase “a while” and the adverb awhile are virtually interchangeable in a sentence, though you should precede the two-word form with the word for: “I think I’ll sit here for a while” and “I think I’ll sit here awhile” mean the same thing.
14. Cannot/can not: Cannot is virtually the only proper alternative. The second usage is wrong except in the correct awkward construction in the sentence “I can not go,” meaning “I can decide not to go.”
15. Everyday/every day: The one-word form is an adjective meaning “ordinary,” used to describe something usual as in “These are my everyday clothes.” The two-word phrase, an adverb, is used in such sentences as “I go there every day” to explain how something is done.
16. Everyone/every one: To refer to everybody, use one word: “Everyone’s a critic.” To emphasize a single individual or item, use two words: “Every one of them is broken.”
17. Everything/every thing: Everything is the default choice: “You’ve ruined everything.” The two-word adjective-noun form is usually divided by an additional adjective: “Every little thing she does is magic.”
18. Maybe/may be: The first choice is an alternative to the adverb perhaps; the second is a verb phrase used in such sentences as “It may be that she was right after all.”
19. Overtime/over time: As one word, this means work done beyond a regular shift: “I’ve worked overtime several days this week.” As two words, it refers to the passage of time: “Over time, we’ve seen dramatic changes.”
20. Sometime/some time: The one-word form is an adverb describing vagueness about when something will happen, as in “I’ll get around to it sometime.”