We have heard that the principal has decided who the prize winner will be and will announce the names in the assembly today.
I thought it should be “whom” because it seems to me that this should be objective but turned out I was wrong…can someone help? :shock:
“Who the prize winner will be” is a clause. The subject of the clause is “the prize winner”, and the pronoun “who” refers to the prize winner, so it will be in the subject case.
You can get to this by replacing things with pronouns.
The principal has decided [who the prize winner will be].
=> The principal has decided him. (Wrong :x )
=> The principal has decided it. (Right 
)
Who/whom has the principal decided? (Bad :evil: )
What has the principal decided? (Good 
)
So, you see that the principal is not deciding [who]; he is deciding [who the prize winner will be]. You can pose that as an echo question: “The prize winner will be who?”
But…do you say “the prize winner will be her” or “the prize winner will be she”?
Hi cooliegirly,
In your sentence:
you can replace who with she to read: the principal has decided who (she)will be the prizewinner .
Alan
Now you’re opening a can of worms! 
Strictly by the grammatical rules of the 19th century, it should be, “The prize winner will be she,” because “she” is not the direct object. The problem is that for centuries there has been a tendency in English to use object pronouns after the verb, even if they aren’t objects. So, although older grammar rules tell us that we should say, “The prize winner will be she,” most English speakers think that sounds strange and would prefer to say, “The prize winner will be her.”
Ah.
Hi cooliegirly,
The whole point (and obviously my point hasn’t been made) is that you would say: She will be the prizewinner
Alan
That makes sense. I see. Thank you both.