Subject/Object Question

[Natalia]

Hello. Look please at the sentence below and tell me if "who" functions here as the subject of the question or object of the question. Thanks.

Who is your favourite singer?

I'm confused because I can answer either: "Shakira is my favourite singer" or "My favourite singer is Shakira"

[Daniel]

It's a predicate nominative, which is neither.

It's not the subject, because the subject controls agreement:

Who are you?
Who is he?
Who are they?
Who am I?

But it's not an object because it's not in the accusative case. For someone whose dialects has 'whom', it would not be whom:
Who is your favorite singer?
Whom* is your favorite singer?

This can be verified by historical documents when who/whom was more common, or in various languages (German, Latin, etc.) where the distinction is still used.
(There is an interesting case in Arabic where such sentences, when there is an overt verb [most of the time 'be' is omitted and looks just like English except the missing verb], actually would use the accusative case, suggesting it's an object, not like in English.)

In short, it is an equational-type sentence like "x=y", where the copula 'be' links a subject to a predicate nominal that is a renaming of that subject.

[MrChiLambda]

Sounds to me as if one sentence is an expression of excitement and the other is a type of forethought?

Saying the singers name first suggests to me that brain retrieval has a node which is focused on the singer first.

Saying the singers name last suggest to me there is an importance placed on the individual, who then reaches further into their memory to retrieve more information as an extension of themselves, rather than a core part of their philosophy.

So, perhaps it is an indication of influence?

[Copernicus]

Hello. Look please at the sentence below and tell me if "who" functions here as the subject of the question or object of the question. Thanks.

Who is your favourite singer?

I'm confused because I can answer either: "Shakira is my favourite singer" or "My favourite singer is Shakira"

The word who is the subject of the question.  The phrase your favorite singer is a noun phrase that plays roughly the same grammatical relation to the copular sentence as that of a verb phrase in a non-copular sentence.  When you answer the question, you are using different sentence structures to place emphasis in different constituents.  So, you can leave Shakira in the default topic position, or you can front (essentially topicalize) the predicate phrase to first position and drag the copula along in its wake.  Copular verbs tend to behave roughly like auxiliary verbs syntactically in all of the languages I am familiar with.