Both “someday” and “some day” are established. Someday is a one-word adverb referring to an unknown future time, while the open form also exists and can be the ordinary determiner-plus-noun construction “some day.”
ReviewedEvidence2 sourcesSectionEnglish Usage
Quick answer
“Someday” is an established adverb; “some day” is also valid, especially when “some” directly modifies “day.”
Key details
Confusion Familyword-boundary-variant
Rule Familyword-boundary
Termssome day, someday
Further guidance
Core difference
“Someday” is an established one-word adverb for an unknown future time; “some day” is also an established two-word expression and can additionally function as determiner plus noun.
Usage Boundary
Do not invent a universal meaning difference when both forms refer to an unspecified future day; follow the governing style and use the open form when “some” directly determines the noun “day”.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
Both “insofar as” and the open form “in so far as” are established ways to express “to the extent that.” The closed form is a dictionary headword, while references also record the open construction, so house style and consistency can determine the choice.
“Anyone” is one word when it means any person. “Any one” is written as two words when the construction refers to a single member of a larger set, including the common pattern “any one of …”.
For the ordinary verb or noun meaning “check again,” major dictionaries use the hyphenated form “double-check.” An open “double check” can arise in other structures and has a specialized chess sense, so it should not be treated as a universal substitute; the closed “doublecheck” is not the general dictionary form used here.
“Everyone” is one word when it means every person. “Every one” is two words when one is a separate element referring to each individual member of a set, especially in constructions such as “every one of them.”
The standard expression is written “no one,” and Cambridge also records the hyphenated variant “no-one.” The fully closed spelling “noone” is not the accepted form in the cited grammar guidance.
“Someone” is the standard one-word pronoun for an unspecified person. The two-word sequence “some one” is possible when “one” remains a separate word and the construction singles out an individual member of a group, so the forms should not be treated as freely interchangeable.
