Hun = to them (indirect object) Hen = them (direct object) In our sentence them is indirect. I give them an apple = I give an apple to them, therefore hun is required.
In the notes, they describe hen/hun as for people, and not used for inanimate objects. Would you still use hen/hun for animals? E.g. if you are giving the apple in this sentence to a pair of horses?
Good question! It's does not work for animals (apart from the possessive form of 'hun').
Can hun and hen be a singular neutral pronoun like they them in english or is it only plural