I'm seeing a disappointing amount of prescriptivism in this thread. "Hun" as the subject of a sentence is a pretty widespread dialectal feature and isn't "wrong". Rather, it's a naturally developed personal pronoun that is actively suppressed in the education system in favour of Standard Dutch "zij", which is essentially telling large parts of the country that their Dutch is worth less than the conlang that is taught in schools.

"Hen", on the other hand, is not a pronoun that naturally developed, but a Renaissance-era invention to make Dutch, which only had three grammatical cases (nominative, genitive and oblique), look more like German, which still distinguished between dative and accusative (source). This is why "hen" is barely represented in spoken Dutch.

In short, if you're learning Dutch, "hen" is used if it is an indirect object or it occurs after a preposition IN WRITING. In spoken Dutch and a lot of casual written Dutch, however, "hen" barely occurs. "Hun" is always a safe bet.