The subject of a verb can be seen
from its conjugation, therefore it's not necessary to keep the pronoun unless you wish to
emphasise it.
In fact, you should only keep the nominative pronoun in a sentence to emphasise something,
and it's wrong to keep it otherwise.
- hogy vagy? how are you?
- te hogy vagy? and how are you?
- itt van a barátnője here is his girlfriend
- itt van az ő
barátnője here is his girlfriend
- ott van a barátom there is my friend
- ott van az én barátom, hol van a tied? there is my friend, where is yours?