我不知道她是谁 用英语怎么说？

I don't know who/whom she is.口语中可用whom,正式的书面语用who.宾语从句中是陈述句语序即:主语+动词+其它.

i don't know who she is.
who she is作为定语从句,需要正常语序

I don't konw who she is .

I don't know who she is. 定语从句在否定句和疑问句都要用陈述句语序。

I don't know whom she is.
whom是she is 的宾语,用宾格

I don't know whom she is.
(在现在的英语中对于who和whom的要求不严,和老外说话who和whom都行.)