Is there a comma after "when", "sometime", "sometime"?
Let us immediately determine that the words once upon a time и someday are indefinite adverbs. In a sentence, they can play the role of a circumstance and are not subject to comma separation, for example:
Once I planted a cherry.
Someday we will meet with you.
But in the context, after these words, there may be introductory words or sentences that clarify the members of the sentence, which are separated by commas, and then after the word "sometime", someday "commas appear:
Once upon a time, it seems last year, we were here.
Someday, perhaps, we will meet again.
The word is different when, which, as a relative pronominal adverb, goes to the service of unions in a complex sentence. If the subordinate clause is after the main clause, then we must put a comma before the union word "when", for example:
I figured out when is the best time to plant cucumbers in the spring.
After the union word "when", there can also be separate members of the sentence or introductory words:
I felt the illusion of what was happening when, as if on purpose, it began to darken quickly.
When, I think out of mischief, he began to speak quietly, we began to ask questions.
The words "when", "sometime", "someday" are used to indicate the time, and, as a rule, a comma is not put after them.
When he was little. Once the rivers were full of fish. We'll go to Disneyland someday.
Commas are used when listing, as well as when using introductory words.
When will the grapes ripen?
Once upon a time the youth gave way to the old.
Someday my dream will come true.
I was born when tulips were blooming.
Before, when I was younger, my acquaintances loved to talk to me.
When it got dark, guests came.
Someday, but not now, love will arise.
When, if you can, tell me, will it rain?
Once upon a time, it was a long time ago, people knew how to fly.
These words are not introductory, commas are not highlighted, unless other conditions are present.
The word "when" is a union, it is preceded by a comma in a complex sentence. After the conjunction "when" a comma is placed, if the conjunction comes next.
The words "sometime", "someday" refer to adverbs, do not require commas after themselves, if the sentence is no longer complicated.
If after these words comes, for example, an introductory word, then the comma must be mandatory.
Example sentence: Someday, perhaps many years from now, I will settle by the sea and meet the dawn every day.
Sometime, probably after the summer holidays, my older brother will return to his home.