I understand that when "those" is referred to as the subject of the verb we use "whom", and when "they" is the object we use "who".
But consider the following sentence,
This is being funded by future taxpayers, many of whom are yet to be born.
Are ‘they’ the object of "are" (to be), in which case ‘they’ should be "who"? Or are ‘they’ the subject of some other verb? On first writing, "whom" seemed correct, but, on looking again, I think it should be "who" because I can’t see what verb ‘they’ could be a subject of.
This was not answered here: Many of who or many of whom?
That question was wrongly marked as a duplicate (IMO) and not answered in the linked question (AFAICS). My question is essentially the same.
In ‘many of whom’, ‘whom’ is the object of a preposition (the preposition being ‘of’). As an object, it takes objective case. In formal writing, ‘whom’ is used as the object form, but informally, many people use ‘who’ for both subject and object.
Some other pronouns that have two different forms, a subject form and an object form, are
If someone isn’t sure if ‘who’ is an object or a subject in a particular sentence, they can often work it out by substituting a different pronoun they are more familiar with.