As far as I was taught, that and which can be used when referring to people, animals and inanimate things, whereas who and whom should only be applied to human beings. The rule does not apply strangely enough to whose, any longer, although I suppose we should use its or theirs of animals and inanimate objects.
I know that language changes over time but I can't help thinking that its current change is accelerated by a generation educated in the 1980s, '90s and early 2000s when 'rules' of grammar and syntax were generally ignored by teachers in favour of 'creativity' and 'authenticity' of expression. We now have generations of teachers who don't know what used to be 'correct' and find it difficult to learn and pass on to their pupils.
Also, written texts are riddled with poor grammar so reading, which used to be a good way to access good models of language use, doesn't help any more.
Please don't tell me that they have to teach grammar in schools today; they do, but whoever devised the curriculum did it in such a way that most children, having to learn technical stuff which was previously O & A level standard, probably lose the will to live after a few lessons
I despair at the corruption of the English language. I don't understand how or why this is happening. There are so many phrases creeping in which are incorrect. Even BBC English is degenerating. Perhaps this is simply a reflection of the general decline in all standards?