Questions & Answers about Un sobre vacío cuenta como voto en blanco.
Because sobre is a masculine noun in Spanish when it means envelope, so it takes un.
- un sobre vacío = an empty envelope
Be careful: sobre can also be a preposition meaning about / on / over, but in this sentence it is clearly the noun envelope.
In Spanish, adjectives often come after the noun, especially when they describe a physical or objective quality.
So:
That word order is the normal one here. Putting the adjective before the noun would sound unusual or more stylistic.
Also, the adjective agrees with the noun:
- un sobre vacío
- una caja vacía
- unos sobres vacíos