I have a sentence:
They are planning to have holidays soon.
What will be the question?
Who IS planning to have their holidays soon?
Who ARE planning to have their holidays soon?
It would be who is planning…?
You can find the explanation here (TheGrammarExchange)
“When ‘who’ and ‘what’ are used to ask for the subject of a clause,
they most often have singular verbs, even if the question expects a
Who is working tomorrow? Phil, Lucy and Shareena (are working
When “who” and ‘what’ are used to ask for the complement of a clause,
they can have plural verbs.
Who are your closest friends?
So the grammatical rule would be that when “who” is not followed by a
noun that refers to it, the verb is singular. However, when there is a
plural noun that serves as the predicate nominative for “who,” the
verb is plural.
Who speaks Spanish in this class? All thirty of us, teacher.
Who is voting for incumbent? The whole town, all 50,000 of us.
Who wants to win the lottery? Everybody!
Who are those students? Who are the people who are voting for the
incumbent? Who are the people who want to win the lottery?