Learn the theory
The difference between hun and hen in Dutch is not even that clear for many native speakers. There are basically two situations in which you use "hun". The first one is really easy:
1. Hun as a possessive pronoun
If something belongs to multiple people, you use "hun".
Dit is hun auto.
Hun dochter heet Julia.
Ik heb hun huis nog niet gezien.
Now this doesn't sound that complicated right? And it is the most important and most common use of "hun".
2. Hun as an indirect object without a preposition
Ok, here it gets confusing, even for many native speakers. Let's look at an example of an indirect object.
Ik geef hun het boek.
I give them the book.
Ik geef het boek aan hen.
I give the book to them.
So, as you can see, the indirect object is "hun" when you don't use a preposition but you could use a preposition. If you choose to use a preposition, you use "hen".
It is important to see that in the examples below we see a direct object, not an indirect object. This means you can only use "hen".
Ik zie hen.
I see them.
Hij tekent hen.
He draws them.
Zij ontslaat hen.
She fires them.
You could also use ze.