question about introductionary phrases

[market]

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.)

[Casiopea]

Welcome.

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.