The pen is yours,mine is in my bag.这句话的主语为什么是"mine"

The book is yours is in my bag 有 mine,Susan is a beauting gi 为什么 Whose school bag is heavier,yours or mine?school bag 后面是 The bag is in the drawer is mine改错 This is my bag.That is——后面应该是mine还是yours found in the park,the bag is mine. All that is yours is rightfully mine! And mine it'll be! 这句话 This bag is mine The bike in the room is mine.“in the room”是修饰mine的定语吗? The pen.He is writing is mine的意思 the pen is on my desk .it is not mine (定语从句的复合句） 划线部分提问 mine,The red pen is mine. Oh,there is a picture of mine on the wall.的mine为什么不是my