I am a teacher.(主格作主语)我是一名老师
Please call me Jim. (宾格作句子的宾语)请叫我吉姆
Who is knocking at the door? It's me.(宾格作句子的表语)谁在敲门?是我
My brother is an engineer.(形容词性物主代词作定语)我哥哥是工程师。
Ours is a big country.(名词性物主代词作主语)我们的国家很大。
You use yours and I use mine.(名词性物主代词作宾语)你用你的,我用我的