Dear Poonam, as far as I understand your question, you are very sure that "Who are you calling?" is wrong, but you are seeking the grammatical reason behind it being wrong. If I am right, kindly read ahead. In the later part, I have given certain grammar rules which may be of use to you. However, in case you aren't looking for the grammatical reason, you may find it difficult to understand.
As per the grammatical rules:
Simple sentences = Subject + (Helping verb + Main Verb) + Object
Question 1. (“Yes/No” questions) = Helping Verb + Subject + Main Verb + Object
Question 2. (“wh” questions) = “wh word (who, when, what, etc)” + Helping Verb + Subject + Main Verb + Object.
(In the second case, certain entities can be omitted as per the context)
1. Helping Verb + Subject + Main Verb + Object
- Do you read books?
- Have you read this book?
2. “wh word (who, when, etc)” + Helping Verb + Subject + Main Verb + Object.
- a. Who are you? (No main Verb, No object)
- b. Who is playing? (No Subject, No object)
- c. Who is playing cricket? (No Subject)
- d. What are you doing? (No object)
- e. When are you going there? (all entities)
One needs to understand when and in what context which element has to be omitted and when the formula is to be used as it is.
Taking help of Hindi language:
“Who” questions can be of two types:
1. Where a verb is being done (koi kaam ho raha hai) as in examples 2.b and c.
2. Where an existence is questioned (kisi cheez ka hona “tha, thi, hai, hain, hoga, honge”) as in example 2.a.
Now, whenever an existence is questioned, the subject is a must whose existence is questioned (“you” in example a), but no verb is done (no main verb) and hence the object is not required.
- Who is calling = kaun call kar raha hai?
- Who are you = aap kaun ho?
- Who are you who is calling = aap kaun ho jo call kar rahe ho?
Note that “jo call kar rahe ho” is irrelevant; “aap kaun ho” is sufficient.
So the key is the formulae mentioned above. Remember that a few of the elements can be omitted as per the context, but the order can't be changed, and nothing other than prepositions, articles, etc., can be added.
Taking your example “Who are you calling”:
- Who are you = existence question
- Who is calling = verb (calling) is being done, so the respective formula is to be used.
Both can't be combined unless a conjunction is used to make it a compound sentence made of two or more simple sentences.
I am sure you must have found it difficult to understand and boring as well. But, this doubt comes under the topic “structuring of sentences,” which is a very basic but very important concept. Generally, speakers know the correct order simply by being in an English-speaking environment, but those who are completely unaware need to be taught this. That doesn't, in any way, mean that you are one of those. What I mean is that it's one of the very basics, and because we don't come across grammatical rules, we may face exceptions and may not understand the correct use because of general and popular use of certain words...
Hope it solved your query...
If it did, you can get in touch for any other guidance. Else, I apologize.
Good day!!
Regards, Shipra