Who is calling Ali?, Can you guess Ali is subj or obj?

[atabitaraf]

John is calling Ali.
1. Who is calling Ali? (answer: John)
2. Who is calling John? (answer: Ali)
3. Whom is calling John? (answer: Ali)
But I think we can't say:
4. Whom is calling Ali? (answer: John)
Because 'whom' is only for objects. Am I right?

[Preceptor]

'Who is calling Ali?' This asks the identity of the person calling Ali.

'Who is calling, Ali?' This asks Ali the identity of the person calling.

'Whom is Ali calling?' This asks the identity of the person Ali is calling.

'Whom is Ali calling, John?' This asks John the identity of the person Ali is calling.

'Who' is nominative; 'whom' is objective. You cannot use 'who' as the direct object, and you cannot use 'whom' as the subject.

[atabitaraf]

'Who is calling Ali?' This asks the identity of the person calling Ali.

'Who is calling, Ali?' This asks Ali the identity of the person calling.

'Whom is Ali calling?' This asks the identity of the person Ali is calling.

'Whom is Ali calling, John?' This asks John the identity of the person Ali is calling.

'Who' is nominative; 'whom' is objective. You cannot use 'who' as the direct object, and you cannot use 'whom' as the subject.

Yes, but my questin is something else.
The sentence of which we have some questions is this: John is calling Ali.
My question is that why the structure is the same whether the answer is John (sunject) or Ali (object)
Please see my post again. There isn't any comma in.

[5jj]

John is calling Ali.
1. Who is calling Ali? (answer: John)
2. Who is calling John? (answer: Ali) If the situation is that John is calling Ali, then you cannot ask this question. Nobody is calling John.
3. Whom is calling John? (answer: Ali) Incorrect English. - We could ask, "Whom is John calling?" - "Ali". However, most people these days would ask, 'Who is John calling?'
But I think we can't say:
4. Whom is calling Ali? (answer: John)This is incorrect English
Because 'whom' is only for objects. Am I right? Yes, and it's very formal.

5

[Soup]

My question is that why [is] the structure is the same whether the answer is John (subject) or Ali (object)

'John' and 'Who" are the subject (SUBJ), 'whom' is the object (OBJ):

SUBJ + VERB + OBJ
Who is calling Ali?
John is calling Ali.
John is calling whom?
Who is calling whom?

OBJ + AUX + SUBJ + VERB
Whom is John calling?
Whom is who calling?

[5jj]

I agree with everything Soup wrote in her last past. I merely note that "Whom is who calling?", while grammaticaly correct, is very unlikely to be seen/heard, in my opinion.

[Tdol]

My question is that why the structure is the same whether the answer is John (sunject) or Ali (object)

It's not- only the first question is correct in your post.