i don't know that where you are这句话对吗? that 什么时候可省?请问你说的第3句是不是主语从句?

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。

i don't know that where you are
这句子不对。把that 省。一般后跟特殊疑问句不用that，一般疑问句用if /whether.陈述句用 that或不用均可。
我是从事英语的，乐意为你解答。

这句话正确,这是个宾语从句.
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.)