如果关系代词that在从句中做宾语，就可以省略。如： the flowers （that） I bought yesterday.... that指代flowers，在从句中做bought的宾语，就可以省略。 做主语的话就不能省。如： All that glitters is not gold. 发光的不都是金子。 that在从句中做glitter的主语，所以不能省。
there's nothing I can do about it.
解析：这个句子nothing后面省略了引导词that，that在句子中引导定语从句，由于that在定语从句中做宾语，所以that可以省略。
Is there anything that you are allergic to?
解析：这句话的that不能省略
当先行词是anything nothing something everything等
后面的连接词必须是that且不可以省略.
综合归纳：宾语从句中that一般情况可省 ，但也并不是任何情况下都可以省略.
宾语从句
1． 定义：用作宾语的从句叫宾语从句.
2． 构成：关联词+简单句
从属连词that.如：
He told us that he felt ill.他对我们说他感到不舒服.
I know he has returned.我知道他已经回来了.(that省略)
注:宾语从句中that一般情况可省 ，但that在引导宾语从句时也并不是任何情况下都可以省略.在以下情况下，that不能省略.
1． Everybody could see what happened and that Tom was frightened.(and连接两个宾语从句，that宾语从句放在and的后面时，that不能省略.)
大家都会看出所发生的事情并知道Tom非常害怕.
2．I know nothing about him except that he is from the south.(that引导的宾语从句作介词宾语时，that不能省略.)
对他我一无所知，只知道他是南方人.
3．That he ever said such a thing I simply don’t believe.(that从句位于句首时，that不可省略.)
我简直不相信他曾说过这样的话.
4． We decided，in view of his special circumstances，that we would admit him for a probationary period.(主句谓语动词与that从句之间有插入语，that不可省略.)
