Student or Learner
can the word (once) be a introductionary phrases. example:
1."Once those cars get close ( ) Thoes kids won't stand a chance."
(If it is a introductionary phrases, then yes there goes a comma.)
2."Once everyone finds out about who you really are ( ) they'll want to hurt you."
(This one too. If it is a intro phrase, then yes there goes a comma.)
Yes, it can, and you have a choice when it comes to using a comma:
When to Use a Comma
Introductory elements often require a comma, but not always. Use a comma in the following cases:
-After an introductory clause. (Does the introductory element have a subject and verb of its own?)
-After a long introductory prepositional phrase or more than one introductory prepositional phrase. (Are there more than five words before the main clause?)
-After introductory verbal phrases, some appositive phrases, or absolute phrases.
-If there is a distinct pause. (When you read the sentence aloud, do you find your voice pausing a moment after the introductory element?)
-To avoid confusion. (Might a reader have to read the sentence more than once to make sense of it?)
When not to Use a Comma
Read more here
All the best.