Have you decided what to do about that job(that) was advertised?

2个从句 第2个的that 是对job的一种解释 也是修饰

如 I know (that)the thing (that) I do is right.
但是你上面第二个that不能去掉 这是代词 明白了吗？
我这个事可以去掉的

是定语从句里的关系代词，代替that job，由于关系代词是在从句中做主语，所以必须保留
I like the book that you bought yesterday我喜欢你昨天买的那本书that 可以省，因为是宾语
I like the book that tells a moving story我喜欢那本讲了一个感人故事的书，不能省，是主语

that是限制性定语从句中的关系代词，代指先行词“job”，由于“was”缺主语,所以要填关系代词。意为“被登了广告的工作”
如：You can drink the juice that was throwed away.

这里that可以用which 替代，是定语从句。
Do you know that book that(which) was published yesterday.

job后面是一句定语从句，that做定语从句的主语指的是job

其实这就是修饰job的一个小定语从句。