请求老师,解答3个英语疑问,was already ,No one ,None ,用of?用’s?

请求老师,解答3个英语疑问,was already ,No one ,None ,用of?用’s?
1.The teacher was already in the classroom when I arrived.
为什麽这句是用一般过去式was already ,而不是现在完成时been already 能用现在完成时been already
2.
None has arrived yet.
No one has arrived yet.
这两句话意思是不是一样?
3
为什麽这里不用of?用’s?
I‘ll be able to afford a week‘s vacation this summer.
有生命的後面,比如“人的”,用“’s’” Mary‘s bag 玛丽的包
week‘s vacation ,为什麽这里星期是没生命的,後面也用’s 没生命的不是应该用of （.a week of vacation