someday is an adverb that modifies a verb to refer to a near or far future event. Its role and function in the English language is to indicate the possibility of something occurring in an unspecified moment in time.
the rules and use cases for the word as an adverb involve understanding that it can be used to indicate anything from an event taking place soon to something that may never occur.
1. She will eventually move out someday.
2. They'll someday be reunited.
3. We'll get better someday.
the main difference to note between the adverb and noun usage of the word is that the adverb usage is always associated with an activity or event in the future, whereas the noun form is only used to refer to a day or time period in the future. With the adverb form, it's important to understand that the amount of time before the event takes place is completely unknown.