"The bus is outside."
Translation:A busz kint van.
The suffix -t shows that a noun is in accusative. So "buszt" can be a direct object in a sentence, while "busz", as seen above, can be a subject.
When exactly can "van" be omitted from a sentence? I tried "A busz kint" and it was incorrect.
when it can be used as an equative; so f.e. in "she is a girl" the "van" can be omitted since the sentence can be seen as an equation (she = girl), in the sentence given above it can't be omitted since you can't equate the bus with the outside (you can see the sentence as "the bus stands outside"; in many languages - f.e. dutch - you can't in such situations say "is" but have to use "lay/sit/stand" etc.)