If you are in Japan with a few other-country friends and hear a voice like that.
a."Who is Chinese ?".
so, You may answer.
b."It's me. I am a Chinese".
This dialogue is translated to Japanese as
so, you can use "私が" to specify yourself from others.
meanwhile when you hear a question and you may answer like that.
c."What is your nationality ?"
d."I am a chinese."
This dialogue is also translated to Japanese as
so you can notice "私は" doesn't specify from others but makes intension who you are.
e."Which is the cucumber in these vegetables ?"
f."It's that. The vegetable is a cucumber."
e. "これらの野菜の中で、どれがキュウリですか ?"
f."それです。 その野菜が キュウリです"
g."What is the vegetable ?" =
h."The vegetable is a cucumber."
g."その野菜は 何ですか ?"