We know that just "the little girl" is "be gai", but when it is the subject it become "Cô gai". Can someone explain or my sentence is true?
Cô gái = does not tell how young the girl is.
Bé gai = the little girl (bé = baby).
As a Vietnamese, I think that we have a flexible age range about "bé gái, cô gái or phụ nữ"
Usually, bé gái is indicated girls under 13-15y.o. Cô gái is for girls from ~15-30y.o. Phụ nữ is for woman over 30 or woman who already get marries
cô gái = young lady
phụ nữ = lady
So they do overlap, but not identical. Note that these are terms to describe a third person. You would never say "hey phụ nữ." When you flirt, you do call "cô gái ơi" but rare.
so what's the common way to address a woman or a girl? Like greeting a child or an older woman. Do all Vietnamese nouns have something like a person system?
Oh, boy, this is complex. Here's the simplified version:
We address people not only based on their age but our age as well.
If we're about the same age, then call them bạn = friend.
If they're younger, call them em. Here's a trick. If you think the lady is older than you, but only by 5-10 years, call her em anyway to make she feels young :-)
if they're older, then anh = male, chị/cô = female.
If they're older than your dad, call them bác.
There are more, but I think these four general rules should be enough to keep you safe.
I think I've read about those somewhere, like ông and bà for very old people, so like you can say cháo ông or something like that to salute an old man as old as your grandfather