The word who functions as pronoun in sentences. It can be either interrogative or relative pronoun.
Who as interrogative pronoun
Who can function as an interrogative pronoun. An interrogative pronoun (who, whom, whose, what, and which) is a pronoun used to ask direct or indirect questions. Examples:
(Comments are in square brackets.)
Who is at the door?
Who do you think will win today’s match?
We’ve to first assess who has skin in the game. [Indirect question. It’s also known as wh-noun clause]
The jury wants to know who will speak first in today’s debate. [Indirect question]
Tom asked who to trust. [Indirect question. It’s a non-finite clause though]
Some mistakenly treat who introducing a noun clause as subordinating conjunction. That’s because, in common parlance, the term subordinating conjunction is associated with any word that joins a dependent clause to an independent clause. But in noun clauses, none of wh-elements (how, when, where, why, who, whom, whose, what, which) is a subordinating conjunction. They are either interrogative adverb, interrogative pronoun (who, for example), or interrogative determiner.
Who as relative pronoun
It can function as relative pronoun as well. A relative pronoun, if you recall, introduces a relative clause, which is placed immediately after the noun phrase it modifies. Examples:
He who finances a venture has the right to make important decisions with respect to that venture. [Relative clause has been underlined]
I haven’t seen Tom, who last attended class nearly a month back, in ages.
Some mistakenly treat who introducing relative clauses as subordinating conjunction. This error though is less common than the one in noun clause. But who, in this role, is a pronoun – relative pronoun – and not conjunction.
If you noticed, who introduces two dependent clauses – relative and noun clause. Since it doesn’t introduce adverb clause, it’s not a subordinating conjunction.
Summary
Who functions as a pronoun (relative and interrogative):
Who are playing from the opposite team? [Interrogative pronoun]
I’m interested in knowing who are playing from the opposite team. [Interrogative pronoun in an indirect question (noun clause)]
Those who are playing from the opposite team would be wary of the depth in our team. [Relative pronoun]
