Why does Duolingo use "ô" and not "đù"?
"Ô" is used primarily in the North while "dù" is used mostly in the South. This Vietnamese course focuses on Northern dialects, especially the Hanoi one. :)
why we dont write (con ô) instead of (Cái ô)?
"con" is the most common classifier for animals (con dê = goat, con ong = bee, con cá = fish, etc). "cái" is the most common classifier for objects and things (cái dù/ô = umbrella, cái ca = mug, cái ly/cốc = glass, etc).