Q: "have an agreed upon" (Very young bilingual children HAVE AN AGREED UPON language they use with the adults they know well.) Why there's this "an" before "agreed" ??? Why not "have agreed upon" ? とはどういう意味ですか?
A: that may or may not be the case, but it’s not what “agreed upon” means here ... here, it simply means “decided beforehand” between the child and the adult, even if it was originally the adult’s idea

“preordained” implies that it was decided by a third party—possibly by fate, or God—beforehand ... it’s too grandiose a term to use here...

you also cannot “agree [something] upon [someone]” in English ... it’s a good question, but we just don’t have a structure like that in English ... two (or more) people mutually “agree upon” something ... which means they “come to an agreement” on / about a topic
Q: it is an agreed upon fact とはどういう意味ですか?
A: an agreed upon fact is something that most people believe to be true. so this sentence is stating that this upcoming statement is believed to be true



Q: We are all agreed on this point .
We all agree on this point .
Which sentence is correct?
A: We all agree on this point.
Q: we all agreed that's correct この表現は自然ですか?
A: QAの全文をご確認ください
Q: i reluctuntly agreed on putting the decision to a vote この表現は自然ですか?
A: @tjstkdn
The gerund could work if you say "to putting" not "on", but I personally prefer the infinitive. I am not sure why it just sounds smoother.