1. So much has changed in the sport since I was a teenager.

2. Since you've finished the work, you may leave anyime

【答】第二句是you've，不是you're。

第一句：much（包括so much）除了是限定词（即：形容词）外，还可以是名词或代词，可以充当句子的主语和宾语。你的句子中，so much充当了主语。

They have done (so) much to advance the cause of peace in the world.  他们已经做了大量工作来促进世界和平事业。（作宾语）

Much has been written on the subject pro and con. 对那问题已有很多赞成和反对的文章。（作主语）

I have (so) much to do to pay my monthly bills.  我应付每月的开支不是容易的。（作宾语）

There is (so) much to eat at the party.  聚会上有很多吃的。（作主语）

第二句：Since you've finished the work，是原因状语从句，不是时间状语从句。