"Who are they hugging?"
Translation:Ketä he halaavat?
As a non-mother-tongue speaker of English, the "who" instead of "whom" is totally putting me off balance, and I have to re-think the sentence to make sure what is ment here.
Using "whom" could be the better choice here - to show English mother tongue speakers, why "ketä" is used and not "kuka". Not to mention us non-mother-tongue speakers who stumble over the fact that the m is missing here.
Ketkä is nominative plural. The nominative plural would be used for multiple subjects of a sentence. Thus "Who's hugging the grandmas?" "The girls": 'Ketkä halaavat mummoja?' 'Tytöt'.
Ketä is partitive singular. It's often used for a single object of a sentence, as one of the translations of formal English 'whom'. Thus "Whom/Who are the grandmas hugging?" "The girl": *'Ketä mummot halaavat?' 'Tyttöä'.
So 'Ketkä he halaavat' doesn't work, because he is marked as the subject, but so is ketkä. It would be a bit like "Who is hugging they?"