When do we use "so" and when do we use "very&

[Helped Wanted]

I was very scared, but the two young girls were so ( or very? ) brave.

When do we use " so " and when do we use "very" in explaining or describing a person or a thing? Thanks ^o^

[Casiopea]

Speakers tend to use "so" and "very" synonymously:

They were very brave.
They were so brave.

Speakers also use "so very" to express intensity:

They were so very brave.

so as intensive.

People sometimes object to the use of so as an intensive meaning “to a great degree or extent”:

We were so relieved to learn that the deadline had been extended.

This usage is most common in informal contexts, perhaps because unlike very, it presumes that the listener or reader will be sympathetic with the speaker’s evaluation of the situation.

Thus you would be more likely to say

It was so unfair of them not to invite you

than to say

It was so fortunate that I didn’t have to put up with your company.

For just this reason, you can sometimes put intensive so to good use in more formal contexts to invite the reader to take the point of view of the speaker or subject:

The request seemed to her to be quite reasonable; it was so unfair of the manager to refuse.

Just remember not to overdo it.

Now that's so very cool, wouldn't you agree?

SOURCE http://www.bartleby.com/64/C001/058.html

[Helped Wanted]

Thank you soooo (so) much, Casiopea! ^0^