who  do  you  want  to   write ?错在哪里呢？

Who do you want to write to?这里的who做介词to的宾语

我同意楼上的意见，write to

I want to write to Tom.对Tom提问就是Who do you want to write to?你想要写给谁？

我赞同二楼的，write to 是固定词组,写信给....

Who do you want to write to?这里的who做介词to的宾语