Who should I ask for?我应该向谁求救?此处who就是宾语；其实who很多时候看似主语都是倒装的,其实还是表语或者宾语,如who are you?
who还可以引导定于从句,在从句中做主语,例如,this is the person who helped me just now.这就是刚才帮助我的那个人；